Illustration: Figure 269Fig. 269. Transverse section through the dorsal region of a young Torpedo embryo to shew the origin of the anterior and posterior roots of the spinal nerves.pr.posterior root of spinal nerve;ar.anterior root of spinal nerve;mp.muscle-plate;ch.notochord;vr.mesoblast cells which will form the vertebral bodies.
Fig. 269. Transverse section through the dorsal region of a young Torpedo embryo to shew the origin of the anterior and posterior roots of the spinal nerves.pr.posterior root of spinal nerve;ar.anterior root of spinal nerve;mp.muscle-plate;ch.notochord;vr.mesoblast cells which will form the vertebral bodies.
Although I have made some efforts to determine the eventual fate of the commissure uniting the dorsal roots, I have not hitherto met with success. It grows thinner and thinner, becoming at the same time composed of fibrous protoplasm with imbedded nuclei, and finally ceases to be recognisable. I can only conclude that it gradually atrophies, and ultimately vanishes.
After the junction of the posterior and anterior roots the compound nerve extends downwards, and may easily be traced for a considerable distance. A special dorsal branch is given off from the ganglion on the posterior root (fig. 275dn). According to Löwe the fibres of the anterior and posterior roots can easily be distinguished in the higher types by their structure and behaviour towards colouring reagents, and can be separately traced in the compound nerve.
So far as has been made out, the development of the spinal nerves of other Vertebrates agrees in the main with that in Elasmobranchii, butno dorsal commissure has yet been discovered, except in the case of the first two or three spinal nerves of the Chick.
In the Chick (Marshall,No.353) the posterior roots, during their early stages, closely resemble those in Elasmobranchii, though their relatively smaller size makes them difficult to observe. They at first extend more orless horizontally outwards above the muscle-plates (as a few of the nerves also do to some extent in Elasmobranchii), but subsequently lie close to the sides of the neural canal. They are shewn in this position infig. 116sp.g.There does not appear to be a continuous crest connecting the roots of the posterior nerves. The later stages of the development are precisely like those in Elasmobranchii.
The anterior roots have not been so satisfactorily investigated as the posterior, but they grow out, possibly by several roots for each nerve, from the ventral corners of the spinal cord, and subsequently become attached to the posterior nerves.
I have observed the development of the posterior roots in Lepidosteus, in which they appear as projections from the dorsal angles of the spinal cord, extending laterally outwards and, at first, having their extremities placed dorsally to the muscle-plates.
Illustration: Figure 270Fig. 270. Transverse section through the posterior part of the head of an embryo chick of thirty hours.hb.hind-brain;vg.vagus nerve;ep.epiblast;ch.notochord;x.thickening of hypoblast (possibly a rudiment of the subnotochordal rod);al.throat;ht.heart;pp.body cavity;so.somatic mesoblast;sf.splanchnic mesoblast;hy.hypoblast.
Fig. 270. Transverse section through the posterior part of the head of an embryo chick of thirty hours.hb.hind-brain;vg.vagus nerve;ep.epiblast;ch.notochord;x.thickening of hypoblast (possibly a rudiment of the subnotochordal rod);al.throat;ht.heart;pp.body cavity;so.somatic mesoblast;sf.splanchnic mesoblast;hy.hypoblast.
The cranial nerves[171]. The earliest stages in the development of the cranial nerves have been most satisfactorily studied, especially by Marshall (No.354), in the Chick, while the later stages have been more fully worked out in Elasmobranchii, where, moreover, they present a very primitive arrangement.In the Chick certain of the cranial nerves arise before the complete closure of the neural groove. These nerves are formed as paired outgrowths of a continuous band composed of two laminæ, connecting the dorsal end of the incompletely closed medullary canal with the external epiblast. This mode of development will best be understood by an examination offig. 270, where the two roots of the vagus nerve (vg) are shewn growing out from the neural band. Shortly after this stage the neural band, becoming separated from the epiblast, constitutes a crest attached to the roof of the brain, while its two laminæ become fused. The relation of the cranial nerves to the brain then becomes exactly the same as that of the posterior roots of the spinal nerves to the spinal cord.
It does not appear possible to decide whether the mode of development of the cranial nerves in the Chick, or that of the posterior roots of the spinal nerves, is the more primitive. The difference in development between the two sets of nerves probably depends upon the relative time of the closure of the neural canal. The neural crest clearly belongs to the brain, from the fact of its remaining connected with the latter when the medullary tube separates from the external epiblast.
It is not known whether the cranial nerves originate before the closure of the neural canal in other forms besides the Chick.
The neural crest of the brain is continuous with that of the spinal cord, and on its separation from the central nervous axis forms on each side a commissure, uniting the posterior cranial nerves with the spinal nerves, and continuous with the commissure connecting together the latter nerves.
Anteriorly, the neural crest extends as far as the roof of the mid-brain[172]. The pairs of nerves which undoubtedly grow out from it are the third pair (Marshall), the fifth, the seventh and auditory (as a single root), the glossopharyngeal, and the various elements of the vagus (as separate roots in Elasmobranchii, but as a single root in Aves). Marshall holds that the olfactorynerve probably also originates from this crest. It will however be convenient to deal separately with this nerve, after treating of the other nerves which undoubtedly arise from the neural crest.
The cranial nerves just enumerated present in their further development many points of similarity; and the glossopharyngeal nerve, as it develops in Elasmobranchii, may perhaps be taken as typical. This nerve is connected by a commissure with those behind, but this fact may for the moment be left out of consideration. Springing at first from the dorsal line of the hind-brain immediately behind the level of the auditory capsule, it apparently loses this primitive attachment and acquires a secondary attachment about halfway down the side of the hind-brain. The primitive undifferentiated rudiment soon becomes divided, exactly like a true posterior root of a spinal nerve, into a root, a ganglion and a nerve. The main branch of the nerve passes ventralwards, and supplies the first branchial arch (fig. 271gl). Shortly afterwards it sends forwards a smaller branch, which passes to the hyoid arch in front; so that the nerve forks over the hyobranchial cleft. A typical cranial nerve appears therefore, except as concerns its relations to the clefts, to develop precisely like the posterior root of the spinal nerve.
Most of the cranial nerves of the above group, in correlation with the highly differentiated character of the head, acquire secondary differentiations, and render necessary a brief description of what is known with reference to their individual development.
The Glossopharyngeal and Vagus Nerves. Behind the ear there are formed, in Scyllium, a series of five nerves which pass down to respectively the first, second, third, fourth and fifth branchial arches.
For each arch there is thus one nerve, whose course lies close to the posterior margin of the preceding cleft; a second anterior branch, forking over the cleft and passing to the arch in front, being developed later. These nerves are connected with the brain by roots at first attached to the dorsal summit, but eventually situated about halfway down the sides. The foremost of them is the glossopharyngeal. The next four are, as has been shewn by Gegenbaur[173], equivalent to four independent nerves, but form together a compound nerve, which we may briefly call the vagus.
Illustration: Figure 271Fig. 271. Views of the head of Elasmobranch embryos at two stages as transparent objects.A. Pristiurus embryo of the same stage as fig. 28 F.B. Somewhat older Scyllium embryo.III.third nerve;V.fifth nerve;VII.seventh nerve;au.n.auditory nerve;gl.glossopharyngeal nerve;Vg.vagus nerve;fb.fore-brain;pn.pineal gland;mb.mid-brain;hb.hind-brain;iv.v.fourth ventricle;cb.cerebellum;ol.olfactory pit;op.eye;au.V.auditory vesicle;m.mesoblast at base of brain;ch.notochord;ht.heart;Vc.visceral clefts;eg.external gills;pp.sections of body cavity in the head.
Fig. 271. Views of the head of Elasmobranch embryos at two stages as transparent objects.A. Pristiurus embryo of the same stage as fig. 28 F.B. Somewhat older Scyllium embryo.III.third nerve;V.fifth nerve;VII.seventh nerve;au.n.auditory nerve;gl.glossopharyngeal nerve;Vg.vagus nerve;fb.fore-brain;pn.pineal gland;mb.mid-brain;hb.hind-brain;iv.v.fourth ventricle;cb.cerebellum;ol.olfactory pit;op.eye;au.V.auditory vesicle;m.mesoblast at base of brain;ch.notochord;ht.heart;Vc.visceral clefts;eg.external gills;pp.sections of body cavity in the head.
This compound nerve together with the glossopharyngeal soon attains a very complicated structure, and presents several remarkable features. There are present five branches (fig. 271B),viz.the glossopharyngeal (gl) and four branches of the vagus, the latter probably arising by a considerably greater number of strands from the brain[174]. All the strands from the brain are united together by a thin commissure (fig. 271B,vg), continuous with the commissure of the posterior roots of the spinal nerves, and from this commissure the five branches are continued obliquely ventralwards and backwards, andeach of them dilates into a ganglionic swelling. They all become again united together by a second thick commissure, which is continued backwards as the intestinal branch of the vagus nerve. The nerves, however, are continued ventralwards each to its respective arch.From the lower commissure springs the lateral nerve, at a point whose relations to the branches of the vagus I have not certainly determined.
With reference to the dorsal commissure, which is almost certainly derived from the original neural crest, it is to be noted that there is a longish stretch of it between the last branch of the vagus and the first spinal nerve, which is probably the remains of a part of the commissure which connected the posterior branches of the vagus, at a stage in the evolution of the Vertebrata, when the posterior visceral clefts were still present. These branches of the vagus are probably partially preserved in the ramifications of the intestinal stem of the vagus (Gegenbaur). The origin of the ventral commissure, continued as the intestinal branch of the vagus, has not been embryologically worked out.
The lateral nerve may very probably be a dorsal sensory branch of the vagus, whose extension into the posterior part of the trunk has been due to the gradual backward elongation of the lateral line[175], causing the nerve supplying it to elongate at the same time (videSection on lateral line).
In the Chick the common rudiment for the vagus and glossopharyngeal nerves (Marshall), which has already been spoken of, subsequently divides into two parts, an anterior forming the glossopharyngeal nerve, and a posterior forming the vagus nerve.
The seventh and auditory nerves. As shewn by Marshall’s and my own observations there is a common rudiment for the seventh and auditory nerves. This rudiment divides almost at once into two branches. The anterior of these pursues a straight course to the hyoid arch (fig. 271A,VII.) and forms the rudiment of the facial nerve; the second of the two (fig. 271A,au.n), which is the rudiment of the auditory nerve, develops a ganglionic enlargement and, turning backwards, closely hugs the ventral wall of the auditory involution (fig. 272).
The seventh or facial nerve soon becomes more complicated. It early develops, like the glossopharyngeal and vagus nerves, a branch, which forks over the cleft in front (spiracle), and supplies the mandibular arch (fig. 271B). This branch forms the præspiracular nerve of the adult, and is homologous with the chorda tympani of Mammalia. Besides however giving rise to this typical branch it gives origin, at a very early period, to two other rather remarkable branches; one of these, arising from its dorsal anterior border, passes forwards to thefront part of the head, immediately dorsal to the ophthalmic branch of the fifth to be described directly. This nerve is the portio major or superficialis of the nerve usually known as the ramus ophthalmicus superficialis in the adult[176].
The other branch of the seventh is the palatine branch—superficial petrosal of Mammalia—the course of which has been more fully investigated by Marshall than by myself. He has shewn that it arises "just below the root of the ophthalmic branch,” and “runs downwards and forwards, lying parallel and immediately superficial to the maxillary branch of the fifth nerve.” This branch of the seventh nerve appears to bear the same sort of relation to the superior maxillary branch of the fifth nerve, that the ophthalmic branch of the seventh does to the ophthalmic branch of the fifth.
Both the root of the seventh and its main branches are gangliated.
The auditory nerve is probably to be regarded as a specially differentiated part of a dorsal branch of the seventh, while the ophthalmic branch may not improbably be a dorsal branch comparable to a dorsal branch of one of the spinal nerves.
The fifth nerve. Shortly after its development the root of the fifth nerve shifts so as to be attached about halfway down the side of the brain. A large ganglion becomes developed close to the root, which forms the rudiment of the Gasserian ganglion. The main branch of the nerve grows into the mandibular arch (fig. 271A,V), maintaining towards it similar relations to those of the posterior nerves to their respective arches.
Two other branches very soon become developed, which were not properly distinguished in my original account. The dorsal one takes a course parallel to the ophthalmic branch of the seventh nerve, and forms, according to the nomenclature already adopted, the portio profunda of the ophthalmicus superficialis of the adult.
The second nerve (fig. 271A) passes forwards, above the mandibular head cavity, and is directed straight towards the eye,near which it meets and unites with the third nerve, where the ciliary ganglion is developed (Marshall). This branch is usually called the ophthalmic branch of the fifth nerve, but Marshall rightly prefers to call it the communicating branch between the fifth and third nerves[177].
Later than these two branches there is developed a third branch, passing to the front of the mouth, and forming the superior maxillary branch of the adult (fig. 271B).
Of the branches of the fifth nerve the main mandibular branch is obviously comparable to the main branch of the posterior nerves. The superficial ophthalmic branch is clearly equivalent to the ophthalmic branch of the seventh. The superior maxillary is usually held to be equivalent to that branch of the posterior nerves which forms the anterior limb of the fork over a cleft. The similarity between the course of this nerve and that of the palatine branch of the seventh, resembling as it does the similar course of the ophthalmic branches of the two nerves, suggests that it may perhaps really be the homologue of the palatine branch of the seventh, therebeing no homologue of the typical anterior branch of the other cranial nerves.
The third nerve. Our knowledge of the development of the third nerve is entirely due to Marshall. He has shewn that in the Chick there is developed from the neural crest, on the roof of the mid-brain, an outgrowth on each side, very similar to the rudiment of the posterior nerves. This outgrowth, the presence of which I can confirm, he believes to be the third nerve, but although he is probably right in this view, it must be borne in mind that there is no direct evidence on the point, the fate of the outgrowth in question not having been satisfactorily followed.
At a very considerably later period a nerve may be foundspringing from the floor of the mid-brain, which is undoubtedly the third nerve, and which Marshall supposes to be the above rudiment, which has shifted its position. It is shewn in Scyllium infig. 271B,III.A few intermediate stages between this and the earliest condition of the nerve have been imperfectly traced by Marshall.
The nerve at the stage represented infig. 271B arises from a ganglionic root, and “runs as a long slender stem almost horizontally backwards, then turns slightly outwards to reach the interval between the dorsal ends of the first and second head cavities, where it expands into a small ganglion.” This ganglion, as first suggested by Schwalbe (No.357), and subsequently proved embryologically by Marshall, is the ciliary ganglion. From the ciliary ganglion two branches arise; one branch continuing the main stem of the nerve, and obviously homologous with the main branch of the other nerves, and the other passing directly forwards “along the top of the first head cavity, then along the inner side of the eye, and finally terminating at the anterior extremity of the head, just dorsal of the olfactory pit.”
The partial separation, in many forms, of the ciliary ganglion from the stem of the third nerve has led to the erroneous view (disproved by the researches of Marshall and Schwalbe) that the ciliary ganglion belongs to the fifth nerve. The connecting branch of the fifth nerve often becomes directly continuous with the anterior branch of the third nerve, and the two together probably constitute the nerve known as the ramus ophthalmicus profundus (Marshall). Further embryological investigations will be required to shew whether this nerve is homologous with the nasal branch of the fifth nerve in Mammalia.
Relations of the nerves to the head-cavities. The cranial nerves, whose development has just been given, bear certain very definite relations to the mesoblastic structures in the head, of the nature of somites, which are known as the head-cavities. Each cranial nerve is typically placed immediately behind the head-cavity of its somite. Thus the main branch of the fifth nerve lies in contact with the posterior wall of the mandibular cavity, as shewn in section infig. 272V. 2ppand in surface view infig. 271; the main branch of the seventh nerve occupies a similar position in relation to the hyoid cavity; and, as Marshall has recently shewn, the main branch of the third nerve adjoins the posterior border of the frontcavity, described by me as the premandibular cavity. Owing to the early conversion of the walls of the posterior head-cavities into muscles, their relations to the nerves are not quite so clear as in the case of the anterior cavities, though, as far as is known, they are precisely the same.
Illustration: Figure 272Fig. 272. Transverse section through the front part of the head of a young Pristiurus embryo.The section, owing to the cranial flexure, cuts both the fore- and the hind-brain. It shews the præmandibular and mandibular head-cavities1ppand2pp, etc.fb.fore-brain;l.lens of eye;m.mouth;pt.upper end of mouth, forming pituitary involution;1ao.mandibular aortic arch;1pp.and2pp.first and second head-cavities;1vc.first visceral cleft;V.fifth nerve;aun.ganglion of auditory nerve;VII.seventh nerve;aa.dorsal aorta;acv.anterior cardinal vein;ch.notochord.
Fig. 272. Transverse section through the front part of the head of a young Pristiurus embryo.The section, owing to the cranial flexure, cuts both the fore- and the hind-brain. It shews the præmandibular and mandibular head-cavities1ppand2pp, etc.fb.fore-brain;l.lens of eye;m.mouth;pt.upper end of mouth, forming pituitary involution;1ao.mandibular aortic arch;1pp.and2pp.first and second head-cavities;1vc.first visceral cleft;V.fifth nerve;aun.ganglion of auditory nerve;VII.seventh nerve;aa.dorsal aorta;acv.anterior cardinal vein;ch.notochord.
Anterior nerve-roots in the brain.
During my investigations on the development of the cranial nerves I was unable to find any roots comparable with the anterior roots of the spinal nerves, and propounded an hypothesis (suggested by the absence of anterior spinal roots in Amphioxus[178]) that the head and trunk had become differentiated from each other at a stage when mixed motor and sensory posterior roots were the only roots present, and I supposed the cranial and spinal nerves to have beenindependentlyevolved from a common ground form, the resulting types of nerves being so different that no roots strictly comparable with the anterior roots of spinal nerves were to be found in the cranial nerves.
The views put forward by me on this subject, though accepted by Schwalbe (No.357), have in other quarters not met with much favour. Wiedersheim holds that it is impossible to believe that the cranial nerves are simpler than the spinal nerves. Such simplicity, which is clearly not found, I have never asserted to exist; I have only stated that the cranial nerves, in acquiring the complicated character they have in the adult, do not develop anterior roots comparable with those of the spinal nerves. Marshall also strongly objects to my views, and has made some observations for the purpose of testing them, leading to some very interesting results, which I proceed to state, and I will then explain my opinion concerning them.
The most important observation of Marshall on this subject concerns the sixth nerve. In both the Chick and Scyllium he has detected a nerve (the first development of which has unfortunately not been made out) arising by a series of roots from the base of the hind-brain. By tracing this nerve to the external rectus muscle of the eye he has satisfactorily identifiedit as the sixth nerve. “Neither in the nerve nor in its roots are there any ganglion cells." This nerve he finds to be placed vertically below the roots of the seventh nerve; and it is not visible till much later than the cranial nerves above described.
In addition to this nerve Marshall has found, both in the third nerve and in the fifth nerve, a series of non-gangliated roots, which arise in a manner not yet satisfactorily elucidated, considerably later than, and in front of, the main roots. These roots join the gangliated roots on the proximal side of the ganglion or in the ganglion[179]; and Marshall believes them to be homologous with the anterior roots of spinal nerves, while he holds the sixth nerve to be an anterior root of the seventh nerve.
In addition to these nerves Marshall holds certain ventral roots, which occur in Elasmobranchs close to the boundary of the spinal cord and medulla, and which probably form the hypoglossal nerve of higher types, to be anterior roots of the vagus. It is very difficult to prove anything definitely about these nerves, but, for reasons stated in my work onElasmobranch Fishes, I am inclined to regard them as anterior roots of one or more spinal nerves.
Before attempting to decide how far Marshall’s views about the so-called anterior roots of the seventh, the fifth and the third nerves are well founded it will conduce to clearness to state the characters and relations of the two roots of spinal nerves.
The posterior root is (1) alwayspurely sensory; (2) it always develops a ganglion. The anterior root is (1) always purely motor; (2) it always joins the posterior rootbelowthe ganglion, except in Petromyzon (though not in Myxine) where the two roots are stated to be independent.
How far do Marshall’s anterior and posterior roots of the cranial nerves exhibit these respective peculiarities?
With reference to the sixth and seventh nerves he states “we must regard the sixth nerve as having the same relation to the seventh that the anterior root of a spinal nerve has to the posterior root.” On this I would remark (1) that the posterior root of this nerve is a mixed sensory and motor nerve and therefore differs in a very fundamental point from that of a spinal nerve; (2) the sixth nerve though resembling the anterior root of a spinal nerve in being motor and without a ganglion, differs from the nearly universal arrangement of spinal nerves in not uniting with the seventh.
With reference to the fifth nerve it is to be observed that it is by no means certain that the whole of the motor fibres are supplied by the so-called anterior roots, and that these roots differ again in the most marked manner from the anterior roots of spinal nerves in joining the main root of the nerveabove(nearer the brain), and not as in a spinal nervebelowtheganglion. The gangliated root of the third nerve is purely motor[180], and its so-called anterior roots again differ from the anterior roots of spinal nerves, in the same manner as those of the fifth nerve.
With reference to the glossopharyngeal and vagus nerves I would merely remark that no anterior root has even been suggested for the glossopharyngeal nerve and that the posterior roots of both these nerves contain a mixture of sensory and motor fibres.
In view of these facts, my original hypothesis appears to me to be confirmed by Marshall’s observations.
The fact of all the posterior roots of the above cranial nerves (except the third which may be purely motor) being mixed motor and sensory roots appears to me to demonstrate that the starting-point of their differentiation was a mixed nerve with a single dorsal root; and that they did not therefore become differentiated from nerves built on the same type as the spinal nerves with dorsal sensory and ventral motor roots. The presence of such non-gangliated roots as those of the third and fifth nerves is not a difficulty to this view. Considering that the cranial nerves are more highly differentiated than the spinal nerves, and have more complicated functions to perform, it would be surprising if there had not been developed nonganglionated rootsanalogous to, but not of course homologous with, the anterior roots of the spinal nerves[181].
As to the sixth nerve further embryological investigations are requisite before its true position in the series can be determined; but it appears to me very probable that it is a product of the differentiation of the seventh nerve.
The fourth nerve. No embryological investigations have been made with reference to the fourth nerve. It is possible that it is a segmental nerve comparable with the third nerve, and that the only remnant still left of the segment to which it belongs is the superior oblique muscle of the eye. If this is the case there must have been two præmandibular segments,viz.that belonging to the third nerve, and that belonging to the fourth nerve. Against this view of the fourth nerve is the fact, urged with great force by Marshall, that the superior oblique muscle is in front of the other eye muscles, and that the fourth nerve therefore crosses the third nerve to reach its destination.
The Olfactory nerve. It was shewn in my monograph on Elasmobranch Fishes that the olfactory nerve grew out from the brain in thesame manner as other nerves; and Marshall (No.355), to whom we are indebted for the greater part of our knowledge on the development of this nerve, has proved that it arises prior to the differentiation of the olfactory lobes.
The earliest stages in the development of the nerve have not been made out. Marshall, as already stated, finds that in the Chick the neural crest is continued in front of the optic vesicles, and holds that this fact is stronga priorievidence in favour of the nerve growing out from it. As mentioned above, note onp.456, I cannot without further evidence accept Marshall’s statements on this point. In any case Marshall has not yet been able again to find an olfactory nerve till long after the disappearance of the neural crest. The olfactory nerve at the next stage observed forms an outgrowth of fusiform cells springing on either side from near the summit of the fore-brain; and at fifty hours it ends close to a slight thickening of the epiblast forming the first rudiment of the olfactory pit, with the walls of which it soon becomes united.
Illustration: Figure 273Fig. 273. Section through the brain and olfactory organ of an embryo of Scyllium.(Modified from figures by Marshall and myself.)c.h.cerebral hemispheres;ol.v.olfactory vesicle;olf.olfactory pit;Sch.Schneiderian folds;I.olfactory nerve. The reference line has been accidentally taken through the nerve to the brain;pn.pineal gland.
Fig. 273. Section through the brain and olfactory organ of an embryo of Scyllium.(Modified from figures by Marshall and myself.)c.h.cerebral hemispheres;ol.v.olfactory vesicle;olf.olfactory pit;Sch.Schneiderian folds;I.olfactory nerve. The reference line has been accidentally taken through the nerve to the brain;pn.pineal gland.
The growth of the cerebral hemispheres causes its point of insertion in the brain to be relatively shifted; and on the development of the olfactory lobes (videpp.444,445) it arises from them (fig. 273). In Elasmobranchs there is a large development of ganglion cells near its root. From Marshall’s figures these appear also to be present in the Chick, but they do not seem to have been found in other forms. In both Teleostei and Amphibia the olfactory nerves are at first extremely short.
Marshall holds that the olfactory nerve is a segmental nerve equivalent to the third, fifth, seventh etc. nerves. It has been already stated that in my opinion the origin of the olfactory nerves from the fore-brain, which I hold to be the ganglion of the præoral lobe, negatives this view. The mere factof these nerves originating as an outgrowth from the central nervous system is no argument in favour of Marshall’s view of their nature; and even if Marshall’s opinion that they arise from the neural crest should turn out to be well founded, this fact would not prove their segmental nature, because their origin from this crest would, as indicated in the next paragraph, merely seem to imply that they primitively arose from the lateral borders of the nerve-plate from which the cerebrospinal tube has been formed.
Situation of the dorsal roots of the cranial and spinal nerves. The probable explanation of the origin of nerves from the neural crest has already been briefly given (p.316). It is that the neural crest represents the original lateral borders of the nervous plate, and that, in the mechanical folding of the nervous plate to form the cerebrospinal canal, its two lateral borders have become approximated in the median dorsal line to form the neural crest. The subsequent shifting of the nerves I am unable to explain, and the meaning of the transient longitudinal commissure connecting the nerves is also unknown. The folding of the neural plate must have extended to the region of the origin of the olfactory nerves, so that, as just stated, there would be no special probability of the olfactory nerves belonging to the same category as the other dorsal nerves from the fact of their springing from the neural crest.
Bibliography of the Peripheral Nervous System.
(351)F. M. Balfour. “On the development of the spinal nerves in Elasmobranch Fishes.”Philosophical Transactions,Vol.CLXVI. 1876;videalso,A monograph on the development of Elasmobranch Fishes. London, 1878,pp.191-216.(352)W. His. “Ueb. d. Anfänge d. peripherischen Nervensystems.”Archiv f. Anat. u. Physiol., 1879.(353)A. M. Marshall. “On the early stages of development of the nerves in Birds.”Journal of Anat. and Phys.,Vol.XI. 1877.(354)A. M. Marshall. “The development of the cranial nerves in the Chick.”Quart. J. of Micr. Science,Vol.XVIII. 1878.(355)A. M. Marshall. “The morphology of the vertebrate olfactory organ.”Quart. J. of Micr. Science,Vol.XIX. 1879.(356)A. M. Marshall. “On the head-cavities and associated nerves in Elasmobranchs.”Quart. J. of Micr. Science,Vol.XXI. 1881.(357)C. Schwalbe. “Das Ganglion oculomotorii.”Jenaische Zeitschrift,Vol.XIII. 1879.
Sympathetic nervous system.
The discovery that the spinal and cranial nerves together with their ganglia were formed from the epiblast was shortly afterwards extended to the sympathetic nervous system, which has now been shewn to arise in connection with the spinal andcranial nerves. The earliest observations on this subject were those contained in myMonograph on Elasmobranch Fishes(p.173), while Schenk and Birdsell (No.361) have since arrived at the same result for Aves and Mammalia.
Illustration: Figure 274Fig. 274. Longitudinal vertical section through part of the body wall of an Elasmobranch embryo shewing part of two spinal nerves and the sympathetic ganglia belonging to them.ar.anterior root;pr.posterior root;sy.g.sympathetic ganglion;mp.part of muscle-plate.
Fig. 274. Longitudinal vertical section through part of the body wall of an Elasmobranch embryo shewing part of two spinal nerves and the sympathetic ganglia belonging to them.ar.anterior root;pr.posterior root;sy.g.sympathetic ganglion;mp.part of muscle-plate.
In my account of the development of these ganglia, it is stated that they were first met with as small masses situated at the ends of short branches of the spinal nerves (fig. 275sy.g). More recent investigations have shewn me that the sympathetic ganglia are at first simply swellings on the main branches of the spinal nerves some way below the ganglia. Their situation may be understood fromfig. 274,sy.g, which belongs however to a somewhat later stage. Subsequently the sympathetic ganglia become removed from the main stem of their respective nerves, remaining however connected with those stems by a short branch (fig. 275,sy.g). I have been unable to find a longitudinal commissure connecting them in their early stages; and I presume that they are at first independent, and become subsequently united into a continuous cord on each side.
The observations of Schenk and Birdsell on the Mammalia seem to indicate that the main parts of the sympathetic system arise in continuity with the posterior spinal ganglia: they also shew that in the neck and other parts the sympathetic cords arise as a continuous ganglionic chain. The observations on the topographical features of the development of the sympathetic system in higher types are however as yet very imperfect.
The later history of the sympathetic ganglia is intimately bound up with that of the so-called suprarenal bodies, which are dealt with in another chapter.
Illustration: Figure 275Fig. 275. Transverse section through the anterior part of the trunk of an embryo of Scyllium slightly older than fig. 29 B.The section is diagrammatic in the fact that the anterior nerve-roots have been inserted for their whole length; whereas they join the spinal cord halfway between two posterior roots.sp.c.spinal cord;sp.g.ganglion of posterior root;ar.anterior root;d.n.dorsally directed nerve springing from posterior root;mp.muscle plate;mp´.part of muscle plate already converted into muscles;mp.l.part of muscle plate which gives rise to the muscles of the limbs;nl.nervus lateralis;ao.aorta;ch.notochord;sy.g.sympathetic ganglion;ca.v.cardinal vein;sp.n.spinal nerve;sd.segmental (archinephric) duct;st.segmental tube;du.duodenum;pan.pancreas;hp.d.point of junction of hepatic duct with duodenum;umc.umbilical canal.
Fig. 275. Transverse section through the anterior part of the trunk of an embryo of Scyllium slightly older than fig. 29 B.The section is diagrammatic in the fact that the anterior nerve-roots have been inserted for their whole length; whereas they join the spinal cord halfway between two posterior roots.sp.c.spinal cord;sp.g.ganglion of posterior root;ar.anterior root;d.n.dorsally directed nerve springing from posterior root;mp.muscle plate;mp´.part of muscle plate already converted into muscles;mp.l.part of muscle plate which gives rise to the muscles of the limbs;nl.nervus lateralis;ao.aorta;ch.notochord;sy.g.sympathetic ganglion;ca.v.cardinal vein;sp.n.spinal nerve;sd.segmental (archinephric) duct;st.segmental tube;du.duodenum;pan.pancreas;hp.d.point of junction of hepatic duct with duodenum;umc.umbilical canal.
Bibliography of the Sympathetic Nervous System.
(360)F. M. Balfour.Monograph on the development of Elasmobranch Fishes.London, 1878,p.173.(361)S. L. SchenkandW. R. Birdsell. “Ueb. d. Lehre von d. Entwicklung d. Ganglien d. Sympatheticus.”Mittheil. a. d. embryologischen Instit. Wien.HeftIII.1879.
[152]Whether there is any part of it in many types not so derived requires further investigation, now that it has been shewn by the Hertwigs that part of the system develops from the endoderm in some Cœlenterata. O. Hertwig holds that part of it has a mesoblastic origin in Sagitta, but his observations on this point appear to me very inconclusive. It would be very advantageous to investigate the origin of Auerbach’s plexus in Mammalia.[153]Our knowledge on this subject is especially due to the brothers Hertwig (Nos.320and321), Eimer (No.318), Claus (No.317), Schäfer (No.326), and Hubrecht (No.323).[154]Reichenbach (No.331) holds that the walls of the groove between the two strands of the ventral cords become invaginated and assist in the formation of the ventral cord.[155]“Ueber Entwicklungsgeschichte d. Echiurus.”Arbeit. a. d. zool. Instit. Wien,Vol.III. 1880.[156]VideVol.II.,pp.273, 274.[157]“Ueber Entwicklungsgeschichte von Teredo.”Arbeit. a. d. zool. Instit. Wien,Vol.III. 1880.[158]For the development of the central nervous system in Amphioxus and the Tunicata the reader is referred to the chapters dealing with those two groups.[159]Löwe (No.341) holds that at an early stage of development three regions can always be distinguished in any section of the central canal,viz.(1) a ventral narrow slit, (2) a median enlargement, and (3) a dorsal slit. Such a form can no doubt often be observed, but my own observations do not lead me to attach any special importance to it.[160]This holds true at first for Elasmobranchii, but at a later stage there are present numerous nerve-cells in the white matter, so that the distinction between the white and grey matter becomes much less marked than in higher types; in this respect Elasmobranchii present an approximation to Amphioxus.[161]It is not within the scope of this work to give an account of the histogenesis of the brain; in the statement in the text only a few points, of some morphological importance, are touched on.[162]I have worked out these changes in Elasmobranchii, Amphibia (Salamandra) and Aves.[163]“Ueb. d. Bau d. centralen Nervensystems d. Axolotl.”Zeit. f. wiss. Zool.,Vol.XXV.1875.[164]For the relations of these bodies,videL. Stieda,“Stud. üb. d. centrale Nervensystem d. Knochenfische.”Zeit. f. wiss. Zool.Vol.XVIII.1868.[165]For a full account of this subjectvideEhlers (No.337).[166]Scott states that in the larva of Petromyzon the pituitary body is derived from the walls of the nasal pit;Quart. J. of Micr. Science,Vol.XXI. p.750. I have not myself completely followed its development in Petromyzon, but I have observed a slight diverticulum of the stomodæum which I believe gives origin to it. Fuller details are in any case required before we can admit so great a divergence from the normal development as is indicated by Scott’s statements.[167]“Les Ascidies simples des Côtes de France.”Archives de Biologie expér. et générale,Vol.III. 1874,p.329.[168]A comparison of the mode of development of this septum with that of the septum lucidum with its contained commissures in Mammalia clearly shews that the two structures are not homologous, and that Miklucho-Maclay is in error in attempting to treat them as being so.[169]Remak derived the posterior ganglia from the tissue of the mesoblastic somites, and following in Remak’s steps most authors believed the peripheral nervous system to have a mesoblastic origin. This view, which had however been rejected on theoretical grounds by Hensen and others, was finally attacked on the ground of observation by His (No.297). His (No.352,p.458) found that in the Fowl “the spinal ganglia of the head and trunk arose from a small band of matter which is placed between the medullary plate and epiblast, and the material of which he called the ‘intermediate cord’.” He further states that: “Before the closure of the medullary tube this band forms a special groove—the ‘intermediate groove’—placed close to the border of the medullary plate. As the closure of the medullary plate into a tube is completed, the earlier intermediate groove becomes a compact cord. In the head of the embryo a longitudinal ridge arises in this way, which separates the suture of the brain from that of the epiblast. In the parts of the neck and in the remaining region of the neck the intermediate cord does not lie over the line of junction of the medullary tube, but laterally from this and forms a ridge, triangular in section, with a slight indrawing.” This intermediate ridge gives rise to four ganglia in the head,viz.the g. trigemini, g. acousticum, g. glossopharyngei, and g. vagi, and in the trunk to the spinal ganglia. In both cases it unites first with the spinal cord.I have given in the above account, as far as possible, a literal translation of His’ own words, because the reader will thus be enabled fairly to appreciate his meaning.Subsequently to His’ memoir (No.297) I gave an account of some researches of my own on this subject (No.351), stating the whole of the nerves to be formed as cellular outgrowths of the spinal cord. I failed fully to appreciate that some of the stages I spoke of had been already accurately described by His, though interpreted by him very differently. Marshall, and afterwards Kölliker, arrived at results in the main similar to my own, and Hensen, independently of and nearly simultaneously with myself, published briefly some observations on the nerves of Mammals in harmony with my results.His has since worked over the subject again (No.352), and has reaffirmed as a result of his work his original statements. I cannot, however, accept his interpretations on the subject, and must refer the reader who is anxious to study them more fully, to His’ own paper.[170]The cellular structure of embryonic nerves is a point on which I should have anticipated that a difference of opinion was impossible, had it not been for the fact that His and Kölliker, following Remak and other older embryologists, absolutely deny the fact. I feel quite sure that no one studying the development of the nerves in Elasmobranchii with well-preserved specimens could for a moment be doubtful on this point, and I can only explain His’ denial on the supposition that his specimens were utterly unsuited to the investigation of the nerves. I do not propose in this work entering into the histogenesis of nerves, but may say that for the earlier stages of their growth, at any rate, my observations have led me in many respects to the same results as Götte (Entwick. d. Unke,pp.482-483), except that I hold that adequate proof is supplied by my investigations to demonstrate that the nerves are for their whole length originally formed as outgrowths of the central nervous system. As the nerve-fibres become differentiated from the primitive spindle-shaped cells, the nuclei become relatively more sparse, and this fact has probably misled Kölliker. Löwe, while admitting the existence of nuclei in the nerves, states that they belong to mesoblastic cells which have wandered into the nerves. This is a purely gratuitous assumption, not supported by observation of the development.[171]The optic nerves are for obvious reasons dealt with in connection with the development of the eye.[172]Marshall holds that the neural crest extends in front of the region of the optic vesicle. I have been unable completely to satisfy myself of the correctness of this statement. In my specimens the epiblast along the line of infolding of this part of the roof of the brain is much thickened, but what Marshall represents as a pair of outgrowths from it like those of a true nerve (No. 354, Pl.II.fig.6) appears to me in my specimens to be part of the external epiblast; and I believe that they remain connected with the external epiblast on the complete separation of the brain from it.[173]“Ueber d. Kopfnerven von Hexanchus,” etc.,Jenaische Zeitschrift,Vol.VI.1871.[174]“Ueber d. Kopfnerven von Hexanchus,” etc.,Jenaische Zeitschrift,Vol.VI. 1871.[175]The peculiar distribution of branches of the fifth and seventh nerves to the lateral line, which is not uncommon, is to be explained in the same manner.[176]The two branches of the ramus ophthalmicus superficialis were spoken of as the ram. opth. superficialis and ram. opth. profundus in myMonograph on Elasmobranch Fishes. The nomenclature in the text is Schwalbe’s, which is probably more correct than mine.[177]Marshall thinks that this nerve may be the remains of the commissure originally connecting the roots of the third and fifth nerves. This suggestion can only be tested by further observations.[178]Schneider holds that anterior roots are present in Amphioxus, but I have been unable to satisfy myself of their presence.[179]These non-gangliated roots of the fifth nerve are not to be confounded with the motor root of the fifth nerve in higher types. They appear to form the anterior root of the adult which gives origin to the ramus ophthalmicus.[180]If Marshall’s view about the ramus ophthalmicus profundus (p.461) is correct, the third must still be, as it no doubt was primitively, a mixed motor and sensory nerve.[181]In the higher types, as is well known, the fifth nerve has its roots formed on the same type as a spinal nerve. The fact that this is not the case in the lower types, either in the embryo or the adult, is a clear indication, to my mind, that the mammalian arrangement of the roots of the fifth nerve has been secondarily acquired, a fact which is a most striking confirmation of my views as to the differences between the cranial and spinal nerves.
[152]Whether there is any part of it in many types not so derived requires further investigation, now that it has been shewn by the Hertwigs that part of the system develops from the endoderm in some Cœlenterata. O. Hertwig holds that part of it has a mesoblastic origin in Sagitta, but his observations on this point appear to me very inconclusive. It would be very advantageous to investigate the origin of Auerbach’s plexus in Mammalia.
[153]Our knowledge on this subject is especially due to the brothers Hertwig (Nos.320and321), Eimer (No.318), Claus (No.317), Schäfer (No.326), and Hubrecht (No.323).
[154]Reichenbach (No.331) holds that the walls of the groove between the two strands of the ventral cords become invaginated and assist in the formation of the ventral cord.
[155]“Ueber Entwicklungsgeschichte d. Echiurus.”Arbeit. a. d. zool. Instit. Wien,Vol.III. 1880.
[156]VideVol.II.,pp.273, 274.
[157]“Ueber Entwicklungsgeschichte von Teredo.”Arbeit. a. d. zool. Instit. Wien,Vol.III. 1880.
[158]For the development of the central nervous system in Amphioxus and the Tunicata the reader is referred to the chapters dealing with those two groups.
[159]Löwe (No.341) holds that at an early stage of development three regions can always be distinguished in any section of the central canal,viz.(1) a ventral narrow slit, (2) a median enlargement, and (3) a dorsal slit. Such a form can no doubt often be observed, but my own observations do not lead me to attach any special importance to it.
[160]This holds true at first for Elasmobranchii, but at a later stage there are present numerous nerve-cells in the white matter, so that the distinction between the white and grey matter becomes much less marked than in higher types; in this respect Elasmobranchii present an approximation to Amphioxus.
[161]It is not within the scope of this work to give an account of the histogenesis of the brain; in the statement in the text only a few points, of some morphological importance, are touched on.
[162]I have worked out these changes in Elasmobranchii, Amphibia (Salamandra) and Aves.
[163]“Ueb. d. Bau d. centralen Nervensystems d. Axolotl.”Zeit. f. wiss. Zool.,Vol.XXV.1875.
[164]For the relations of these bodies,videL. Stieda,“Stud. üb. d. centrale Nervensystem d. Knochenfische.”Zeit. f. wiss. Zool.Vol.XVIII.1868.
[165]For a full account of this subjectvideEhlers (No.337).
[166]Scott states that in the larva of Petromyzon the pituitary body is derived from the walls of the nasal pit;Quart. J. of Micr. Science,Vol.XXI. p.750. I have not myself completely followed its development in Petromyzon, but I have observed a slight diverticulum of the stomodæum which I believe gives origin to it. Fuller details are in any case required before we can admit so great a divergence from the normal development as is indicated by Scott’s statements.
[167]“Les Ascidies simples des Côtes de France.”Archives de Biologie expér. et générale,Vol.III. 1874,p.329.
[168]A comparison of the mode of development of this septum with that of the septum lucidum with its contained commissures in Mammalia clearly shews that the two structures are not homologous, and that Miklucho-Maclay is in error in attempting to treat them as being so.
[169]Remak derived the posterior ganglia from the tissue of the mesoblastic somites, and following in Remak’s steps most authors believed the peripheral nervous system to have a mesoblastic origin. This view, which had however been rejected on theoretical grounds by Hensen and others, was finally attacked on the ground of observation by His (No.297). His (No.352,p.458) found that in the Fowl “the spinal ganglia of the head and trunk arose from a small band of matter which is placed between the medullary plate and epiblast, and the material of which he called the ‘intermediate cord’.” He further states that: “Before the closure of the medullary tube this band forms a special groove—the ‘intermediate groove’—placed close to the border of the medullary plate. As the closure of the medullary plate into a tube is completed, the earlier intermediate groove becomes a compact cord. In the head of the embryo a longitudinal ridge arises in this way, which separates the suture of the brain from that of the epiblast. In the parts of the neck and in the remaining region of the neck the intermediate cord does not lie over the line of junction of the medullary tube, but laterally from this and forms a ridge, triangular in section, with a slight indrawing.” This intermediate ridge gives rise to four ganglia in the head,viz.the g. trigemini, g. acousticum, g. glossopharyngei, and g. vagi, and in the trunk to the spinal ganglia. In both cases it unites first with the spinal cord.
I have given in the above account, as far as possible, a literal translation of His’ own words, because the reader will thus be enabled fairly to appreciate his meaning.
Subsequently to His’ memoir (No.297) I gave an account of some researches of my own on this subject (No.351), stating the whole of the nerves to be formed as cellular outgrowths of the spinal cord. I failed fully to appreciate that some of the stages I spoke of had been already accurately described by His, though interpreted by him very differently. Marshall, and afterwards Kölliker, arrived at results in the main similar to my own, and Hensen, independently of and nearly simultaneously with myself, published briefly some observations on the nerves of Mammals in harmony with my results.
His has since worked over the subject again (No.352), and has reaffirmed as a result of his work his original statements. I cannot, however, accept his interpretations on the subject, and must refer the reader who is anxious to study them more fully, to His’ own paper.
[170]The cellular structure of embryonic nerves is a point on which I should have anticipated that a difference of opinion was impossible, had it not been for the fact that His and Kölliker, following Remak and other older embryologists, absolutely deny the fact. I feel quite sure that no one studying the development of the nerves in Elasmobranchii with well-preserved specimens could for a moment be doubtful on this point, and I can only explain His’ denial on the supposition that his specimens were utterly unsuited to the investigation of the nerves. I do not propose in this work entering into the histogenesis of nerves, but may say that for the earlier stages of their growth, at any rate, my observations have led me in many respects to the same results as Götte (Entwick. d. Unke,pp.482-483), except that I hold that adequate proof is supplied by my investigations to demonstrate that the nerves are for their whole length originally formed as outgrowths of the central nervous system. As the nerve-fibres become differentiated from the primitive spindle-shaped cells, the nuclei become relatively more sparse, and this fact has probably misled Kölliker. Löwe, while admitting the existence of nuclei in the nerves, states that they belong to mesoblastic cells which have wandered into the nerves. This is a purely gratuitous assumption, not supported by observation of the development.
[171]The optic nerves are for obvious reasons dealt with in connection with the development of the eye.
[172]Marshall holds that the neural crest extends in front of the region of the optic vesicle. I have been unable completely to satisfy myself of the correctness of this statement. In my specimens the epiblast along the line of infolding of this part of the roof of the brain is much thickened, but what Marshall represents as a pair of outgrowths from it like those of a true nerve (No. 354, Pl.II.fig.6) appears to me in my specimens to be part of the external epiblast; and I believe that they remain connected with the external epiblast on the complete separation of the brain from it.
[173]“Ueber d. Kopfnerven von Hexanchus,” etc.,Jenaische Zeitschrift,Vol.VI.1871.
[174]“Ueber d. Kopfnerven von Hexanchus,” etc.,Jenaische Zeitschrift,Vol.VI. 1871.
[175]The peculiar distribution of branches of the fifth and seventh nerves to the lateral line, which is not uncommon, is to be explained in the same manner.
[176]The two branches of the ramus ophthalmicus superficialis were spoken of as the ram. opth. superficialis and ram. opth. profundus in myMonograph on Elasmobranch Fishes. The nomenclature in the text is Schwalbe’s, which is probably more correct than mine.
[177]Marshall thinks that this nerve may be the remains of the commissure originally connecting the roots of the third and fifth nerves. This suggestion can only be tested by further observations.
[178]Schneider holds that anterior roots are present in Amphioxus, but I have been unable to satisfy myself of their presence.
[179]These non-gangliated roots of the fifth nerve are not to be confounded with the motor root of the fifth nerve in higher types. They appear to form the anterior root of the adult which gives origin to the ramus ophthalmicus.
[180]If Marshall’s view about the ramus ophthalmicus profundus (p.461) is correct, the third must still be, as it no doubt was primitively, a mixed motor and sensory nerve.
[181]In the higher types, as is well known, the fifth nerve has its roots formed on the same type as a spinal nerve. The fact that this is not the case in the lower types, either in the embryo or the adult, is a clear indication, to my mind, that the mammalian arrangement of the roots of the fifth nerve has been secondarily acquired, a fact which is a most striking confirmation of my views as to the differences between the cranial and spinal nerves.
In the lowest forms of animal life the whole surface is sensitive to light, and organs of vision have no doubt arisen in the first instance from limited areas becoming especially sensitive to light in conjunction with a deposit of pigment. Lens-like structures, formed either as a thickening of the cuticle, or as a mass of cells, were subsequently formed; but their function was not, in the first instance, to throw an image of external objects on the perceptive part of the eye, but to concentrate the light on it. From such a simple form of visual organ it is easy to pass by a series of steps to an eye capable of true vision.
There are but few groups of the Metazoa which are not provided with optic organs of greater or less complexity.
In a large number of instances these organs are placed on the anterior part of the head, and are innervated from the anterior ganglia. It is possible that many of the eyes so situated may be modifications of a common prototype. In other instances organs of vision are situated in different regions of the body, and it is clear that such eyes have been independently evolved in each instance.
The percipient elements of the eye would invariably appear to be cells, one end of each of which is continuous with a nerve, while the other terminates in a cuticular structure, or indurated part of the cell forming what is known as the rod or cone.
The presence of such percipient elements in various eyes is therefore no proof of genetic relationship between these eyes, but merely of similarity of function.
Embryological data as to the development of the eye do notexist except in the case of the Arthropoda, Mollusca and Chordata. From such data as there are, combined with study of the adult structure of the eye, it can be shewn that two types of development are found. In one of these the percipient elements are formed from the central nervous system, in the other from the epidermis. The former may be called cerebral eyes. It is probable however that this distinction is not, in all cases at any rate, so fundamental as might be supposed; but that in both instances the eye may have taken its origin from the epidermis. In the eyes in which the retina is continuous with the central nervous system, these two organs were probably evolved simultaneously as differentiations of the epidermis, and continue to develop together in the ontogenetic growth of the eye.
Some of the eyes in which the retina is formed from the epidermis have also probably arisen simultaneously with part of the central nervous system, while in other instances they have arisen as later formations subsequently to the complete establishment of a central nervous system.