Illustration: Figure 256Fig. 256. 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 premandibular and mandibular head cavities1ppand2pp, etc. The section is moreover somewhat oblique from side to side.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.auditory nerve;VII.seventh nerve;aa.roots of dorsal aorta;acv.anterior cardinal vein;ch.notochord.
Fig. 256. 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 premandibular and mandibular head cavities1ppand2pp, etc. The section is moreover somewhat oblique from side to side.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.auditory nerve;VII.seventh nerve;aa.roots of dorsal aorta;acv.anterior cardinal vein;ch.notochord.
Although in the majority of the Vertebrata there is a close connection between the pituitary body and the infundibulum, there is no actual fusion between the two. In Mammalia the case is different. The part of the infundibulum which lies at the hinder end of the pituitary body is at first a simple finger-like process of the brain (fig. 255inf), but its end becomes swollen, and the lumen in this part becomes obliterated. Its cells, originally similar to those of the other parts of the nervous system and even (Kölliker) containing differentiated nerve-fibres, partly atrophy, and partly assume an indifferent form, while at the same timethere grow in amongst them numerous vascular and connective-tissue elements. The process of the infundibulum thus metamorphosed becomes inseparably connected with the true pituitary body, of which it is usually described as the posterior lobe. The part of the infundibulum which undergoes this change is very probably homologous with the saccus vasculosus of Fishes.
The true nature of the pituitary body has not yet been made out. It is clearly a rudimentary organ in existing craniate Vertebrates, and its development indicates that when functional it was probably a sense organ opening into the mouth, its blind end reaching to the base of the brain. No similar organ has as yet been found in Amphioxus, but it seems possible perhaps to identify it with the peculiar ciliated sack placed at the opening of the pharynx in the Tunicata, the development of which was described atp.18. If the suggestion is correct, the division of the body into lobes in existing Vertebrata must be regarded as a step towards a retrogressive metamorphosis.
Another possible view is to regard the pituitary body as a glandular structure which originally opened into the mouth in the lower Chordata, but which has in all existing forms ceased to be functional. The intimate relation of the organ to the brain appears to me opposed to this view of its nature, while on the other hand its permanent structure is more easily explained on this view than on that previously stated. In the Ascidians a glandular organ has been described by Lacaze Duthiers[167]in juxtaposition to the ciliated sack, and it is possible that this organ as well as the ciliated sack may be related to the pituitary body. In view of this possibility further investigations ought to be carried out in order to determine whether the whole pituitary body is derived from the oral involution, or whether there may not be a nervous part and a glandular part of the organ.
The Cerebral Hemispheres. It will be convenient to treat separately the development of the cerebral hemispheres proper, and that of the olfactory lobes.
Although the cerebral hemispheres vary more than any other part of the brain, they are nevertheless developed from the unpaired cerebral rudiment in a nearly similar manner throughout the series of Vertebrata.
In the cerebral rudiment two parts may be distinguished,viz.the floor and the roof. The former gives rise to the ganglia at the base of the hemispheres—corpora striata, etc.—the latter to the hemispheres proper.
Illustration: Figure 257Fig. 257. Diagrammatic longitudinal horizontal section through the fore-brain.3.v.third ventricle;lv.lateral ventricle;lt.lamina terminalis;ce.cerebral hemisphere;op.th.optic thalamus.
Fig. 257. Diagrammatic longitudinal horizontal section through the fore-brain.3.v.third ventricle;lv.lateral ventricle;lt.lamina terminalis;ce.cerebral hemisphere;op.th.optic thalamus.
The first change which takes place consists in the roof growing out into two lobes, between which a shallow median constriction makes its appearance (fig. 257). The two lobes thus formed are the rudiments of the two hemispheres. The cavity of each of them opens by a widish aperture into the vestibule at the base of the cerebral rudiment, which again opens directly into the cavity of the third ventricle (3 v). The Y-shaped aperture thus formed, which leads from the cerebral hemispheres into the third ventricle, is the foramen of Munro. The cavity (lv) in each of the rudimentary hemispheres is a lateral ventricle. The part of the cerebrum which lies between the two hemispheres, and passes forwards from the roof of the third ventricle round the end of the brain to the optic chiasma, is the rudiment of the lamina terminalis (figs.257ltand255trm). Up to this point the development of the cerebrum is similar in all Vertebrata, but in some forms it practically does not proceed much further.
In Elasmobranchii, although the cerebrum reaches a considerable size (fig. 254cer), and grows some way backwards over the thalamencephalon, yet it is not in many forms divided into two distinct lobes, but its paired nature is only marked by a shallow constriction on the surface. The lamina terminalis in the later stages of development grows backwards as a thick median septum which completely separates the two lateral ventricles[168](fig. 263).
There are, it may be mentioned, considerable variations inthe structure of the cerebrum in Elasmobranchii into which it is not however within the scope of this work to enter.
In the Teleostei the vesicles of the cerebral hemispheres appear at first to have a wide lumen, but it subsequently becomes almost or quite obliterated, and the cerebral rudiment forms a small bilobed nearly solid body. In Petromyzon (fig. 253ch) the cerebral rudiment is at first an unpaired anterior vesicle, which subsequently becomes bilobed in the normal manner. The walls of the hemispheres become much thickened, but the lateral ventricles persist.
In all the higher Vertebrates the division of the cerebral rudiment into two distinct hemispheres is quite complete, and with the deepening of the furrow between the two hemispheres the lamina terminalis is carried backwards till it forms a thin layer bounding the third ventricle anteriorly, while the lateral ventricles open directly into the third ventricle.
In Amphibians the two hemispheres become united together immediately in front of the lamina terminalis by commissural fibres, forming the anterior commissure. They also send out anteriorly two solid prolongations, usually spoken of as the olfactory lobes, which subsequently fuse together.
In all Reptilia and Aves there is formed an anterior commissure, and in the higher members of the group, especially Aves (fig. 250), the hemispheres may obtain a considerable development. Their outer walls are much thickened, while their inner walls become very thin; and a well-developed ganglionic mass, equivalent to the corpus striatum, is formed at their base.
Illustration: Figure 258Fig. 258. Brain of a three months’ human embryo: natural size.(From Kölliker.)1. From above with the dorsal part of hemispheres and mid-brain removed; 2. From below.f.anterior part of cut wall of the hemisphere;f´.cornu ammonis;tho.optic thalamus;cst.corpus striatum;to.optic tract;cm.corpora mammillaria;p.pons Varolii.
Fig. 258. Brain of a three months’ human embryo: natural size.(From Kölliker.)1. From above with the dorsal part of hemispheres and mid-brain removed; 2. From below.f.anterior part of cut wall of the hemisphere;f´.cornu ammonis;tho.optic thalamus;cst.corpus striatum;to.optic tract;cm.corpora mammillaria;p.pons Varolii.
The cerebral hemispheres undergo in Mammalia the most complicated development. The primitive unpaired cerebral rudiment becomes, as in lower Vertebrates, bilobed, and at the same time divided by the ingrowth of a septum of connective tissue into two distinct hemispheres (figs.260and261fand2581). From this septum is formed the falx cerebri and other parts.
The hemispheres contain at first very large cavities, communicating by a wide foramen of Munro with the third ventricle (fig. 260). They grow rapidly in size, and extend,especially backwards, and gradually cover the thalamencephalon and themid-brain (fig. 2581,f). The foramen of Munro becomes very much narrowed and reduced to a mere slit.
The walls are originally nearly uniformly thick, but the floor becomes thickened on each side, and gives rise to the corpus striatum (figs.260and261st). The corpus striatum projects upwards into each lateral ventricle, giving to it a somewhat semilunar form, the two horns of which constitute the permanent anterior and descending cornua of the lateral ventricles (fig. 262st).
Illustration: Figure 259Fig. 259. Transverse section through the brain of a rabbit of five centimetres.(After Mihalkovics.)The section passes through nearly the posterior border of the septum lucidum, immediately in front of the foramen of Munro.hms.cerebral hemispheres;cal.corpus callosum;amm.cornu ammonis (hippocampus major);cms.superior commissure of the cornua ammonis;spt.septum lucidum;frx 2.vertical fibres of the fornix;cma.anterior commissure;trm.lamina terminalis;str.corpus striatum;ltf.nucleus lenticularis of corpus striatum;vtr 1.lateral ventricle;vtr 3.third ventricle;ipl.slit between cerebral hemispheres.
Fig. 259. Transverse section through the brain of a rabbit of five centimetres.(After Mihalkovics.)The section passes through nearly the posterior border of the septum lucidum, immediately in front of the foramen of Munro.hms.cerebral hemispheres;cal.corpus callosum;amm.cornu ammonis (hippocampus major);cms.superior commissure of the cornua ammonis;spt.septum lucidum;frx 2.vertical fibres of the fornix;cma.anterior commissure;trm.lamina terminalis;str.corpus striatum;ltf.nucleus lenticularis of corpus striatum;vtr 1.lateral ventricle;vtr 3.third ventricle;ipl.slit between cerebral hemispheres.
With the further growth of the hemisphere the corpusstriatum loses its primitive relations to the descending cornu. The reduction in size of the foramen of Munro above mentioned is, to a large extent, caused by the growth of the corpora striata.
Illustration: Figure 260Fig. 260. Transverse section through the brain of a sheep’s embryo of 2.7cm.in length.(From Kölliker.)The section passes through the level of the foramen of Munro.st.corpus striatum;m.foramen of Munro;t.third ventricle;pl.choroid plexus of lateral ventricle;f.falx cerebri;th.anterior part of optic thalamus;ch.optic chiasma;o.optic nerve;c.fibres of the cerebral peduncles;h.cornu ammonis;p.pharynx;sa.presphenoid bone;a.orbitosphenoid bone;s.points to part of the roof of the brain at the junction between the roof of the third ventricle and the lamina terminalis;l.lateral ventricle.
Fig. 260. Transverse section through the brain of a sheep’s embryo of 2.7cm.in length.(From Kölliker.)The section passes through the level of the foramen of Munro.st.corpus striatum;m.foramen of Munro;t.third ventricle;pl.choroid plexus of lateral ventricle;f.falx cerebri;th.anterior part of optic thalamus;ch.optic chiasma;o.optic nerve;c.fibres of the cerebral peduncles;h.cornu ammonis;p.pharynx;sa.presphenoid bone;a.orbitosphenoid bone;s.points to part of the roof of the brain at the junction between the roof of the third ventricle and the lamina terminalis;l.lateral ventricle.
The corpora striata are united at their posterior border with the optic thalami. In the later stages of development the area of contact between these two pairs of ganglia increases to an immense extent (fig. 261), and the boundary between them becomes somewhat obscure, so that the sharp distinction which exists in the embryo between the thalamencephalon and cerebral hemispheres becomes lost. This change is usually (Mihalkovics, Kölliker) attributed to a fusion between the corpora striata and optic thalami, but it has recently been attributed by Schwalbe (No.349), with more probability, to a growth of the original surface of contact, and an accompanying change in the relations of the parts.
The outer wall of the hemispheres gradually thickens, while the inner wall becomes thinner. In the latter, two curved folds, projecting towards the interior of the lateral ventricle, become formed. These folds extend from the foramen of Munro along nearly the whole of what afterwards becomes the descending cornu of the lateral ventricle.
The upper fold becomes the hippocampus major (cornu ammonis) (figs.259amm,260and261h, and262am). When the rudiment of the descending cornu has become transformed into a simple process of the lateral ventricle the hippocampus major forms a prominence upon its floor.
Illustration: Figure 261.Fig. 261. Transverse section through the brain of a sheep’s embryo of 2.7cm.in length.(From Kölliker.)The section is taken a short distance behind the section represented in fig. 260, and passes through the posterior part of the hemispheres and the third ventricle.st.corpus striatum;th.optic thalamus;to.optic tract;t.third ventricle;d.roof of third ventricle;c.fibres of cerebral peduncles;c´.divergence of these fibres into the walls of the hemispheres;e.lateral ventricle with choroid plexuspl; hcornu ammonis;f.primitive falx;am.alisphenoid;a.orbitosphenoid;sa.presphenoid;p.pharynx;mk.Meckel’s cartilage.
Fig. 261. Transverse section through the brain of a sheep’s embryo of 2.7cm.in length.(From Kölliker.)The section is taken a short distance behind the section represented in fig. 260, and passes through the posterior part of the hemispheres and the third ventricle.st.corpus striatum;th.optic thalamus;to.optic tract;t.third ventricle;d.roof of third ventricle;c.fibres of cerebral peduncles;c´.divergence of these fibres into the walls of the hemispheres;e.lateral ventricle with choroid plexuspl; hcornu ammonis;f.primitive falx;am.alisphenoid;a.orbitosphenoid;sa.presphenoid;p.pharynx;mk.Meckel’s cartilage.
The wall of the lower fold becomes very thin, and a vascular plexus, derived from the connective-tissue septum between the hemispheres, and similar to that of the roof of the third ventricle,is formed outside it. It constitutes a fold projecting far into the cavity of the lateral ventricle, and together with the vascular connective tissue in it gives rise to the choroid plexus of the lateral ventricle (figs.260and261pl).
It is clear from the above description that a marginal fissure leading into the cavity of the lateral ventricle does not exist in the sense often implied in works on human anatomy, in that the epithelium covering the choroid plexus, which forms the true wall of the brain, is a continuous membrane. Theepitheliumof the choroid plexus of the lateral ventricle is quite independent of that of the choroid plexus of the third ventricle, though at the foramen of Munro the roof of the third ventricle is of course continuous with the inner wall of the lateral ventricle (fig. 260s). Thevascular elementsof the two plexuses form however a continuous structure.
The most characteristic parts of the Mammalian cerebrum are the commissures connecting the two hemispheres. These commissures are (1) the anterior commissure, (2) the fornix, and (3) the corpus callosum, the two latter being peculiar to Mammalia.
By the fusion of the inner walls of the hemispheres in front of the lamina terminalis a solid septum is formed, known as the septum lucidum, continuous behind with the lamina terminalis, and below with the corpora striata (figs.255and259spt). It is by a series of differentiations within this septum that the above commissures originate. In Man there is a closed cavity left in the septum known as the fifth ventricle, which has however no communication with the true ventricles of the brain.
In the septum lucidum there become first formed, below, the transverse fibres of the anterior commissure (fig. 255andfig. 259cma), and in the upper part the vertical fibres of the fornix (fig. 255andfig. 259frx 2). The vertical fibres meet above the foramen of Munro, and thence diverge backwards, as the posterior pillars, to lose themselves in the cornu ammonis (fig. 259amm). Ventrally they are continued, as the descending or anterior pillars of the fornix, into the corpus albicans, and thence into the optic thalami.
The corpus callosum is not formed till after the anterior commissure and fornix. It arises in the upper part of the region(septum lucidum) formed by the fusion of the lateral walls of the hemispheres (figs.255and259cal), and at first only its curved anterior portion—the genu or rostrum—is developed. This portion is alone found in Monotremes and Marsupials. The posterior portion, which is present in all the Monodelphia, is gradually formed as the hemispheres are prolonged further backwards.
Illustration: Figure 262Fig. 262. Lateral view of the brain of a calf embryo of 5cm.(After Mihalkovics.)The outer wall of the hemisphere is removed, so as to give a view of the interior of the left lateral ventricle.hs.cut wall of hemisphere;st.corpus striatum;am.hippocampus major (cornu ammonis);d.choroid plexus of lateral ventricle;fm.foramen of Munro;op.optic tract;in.infundibulum;mb.mid-brain;ch.cerebellum;IV.V.roof of fourth ventricle;ps.pons Varolii, close to which is the fifth nerve with Gasserian ganglion.
Fig. 262. Lateral view of the brain of a calf embryo of 5cm.(After Mihalkovics.)The outer wall of the hemisphere is removed, so as to give a view of the interior of the left lateral ventricle.hs.cut wall of hemisphere;st.corpus striatum;am.hippocampus major (cornu ammonis);d.choroid plexus of lateral ventricle;fm.foramen of Munro;op.optic tract;in.infundibulum;mb.mid-brain;ch.cerebellum;IV.V.roof of fourth ventricle;ps.pons Varolii, close to which is the fifth nerve with Gasserian ganglion.
Primitively the Mammalian cerebrum, like that of the lower Vertebrata, is quite smooth. In many of the Mammalia,Monotremata,Insectivora, etc., this condition is nearly retained through life, while in the majority of Mammalia a more or less complicated system of fissures is developed on the surface. The most important, and first formed, of these is the Sylvian fissure. It arises at the time when the hemispheres, owing to their growth in front of and behind the corpora striata, have assumed a somewhat bean-shaped form. At the root of the hemispheres—the hilus of the bean—there is formed a shallow depression, which constitutes the first trace of the Sylvian fissure. The part of the brain lying in this fissure is known as the island of Reil.
The olfactory lobes. The olfactory lobes, or rhinencephala, are secondary outgrowths of the cerebral hemispheres, and contain prolongations of the lateral ventricles, but may however be solid in the adult state. According to Marshall they develop in Birds and Elasmobranchs and presumably other forms later than the olfactory nerves, so that the olfactory region of the hemispheres is indicated before the appearance of the olfactory lobes.
In most Vertebrates the olfactory lobes arise at a fairly earlystage of development from the under and anterior part of the hemispheres (fig. 250olf). In Elasmobranchs they arise, not from the base, but from the lateral parts of the brain (fig. 263), and become subsequently divided into a bulbous portion and a stalk. They vary considerably in their structure in the adult.
Illustration: Figure 263Fig. 263. Section through the brain and olfactory organ of an embryo of Scyllium.(Modified from figures by Marshall and myself.)ch.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.anterior prolongation of pineal gland.
Fig. 263. Section through the brain and olfactory organ of an embryo of Scyllium.(Modified from figures by Marshall and myself.)ch.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.anterior prolongation of pineal gland.
In Amphibia the solid anterior prolongations of the cerebral hemispheres already spoken of are usually regarded as the olfactory lobes, but according to Götte, whose view appears to me well founded, small papillæ, situated at the base of these prolongations, from which olfactory nerves spring, and which contain a process of the lateral ventricle, should properly be regarded as the olfactory lobes. These papillæ arise prior to the solid anterior prolongations of the hemispheres.
In Birds the olfactory lobes are small. In the chick they arise (Marshall) on the seventh day of incubation.
General conclusions as to the Central Nervous System.
It has been shewn above that both the brain and spinal cord are primitively composed of a uniform wall of epithelial cells, and that the first differentiation results in the formation of an external layer of white matter, a middle layer of grey matter (ganglion cells), and an inner epithelial layer. This primitivehistological arrangement, which in many parts of the brain at any rate, is only to be observed in the early developmental stages, has a simple phylogenetic explanation.
As has been already explained in an earlier part of this chapter the central nervous system was originally a differentiated part of the superficial epidermis.
This differentiation (as may be concluded from the character of the nervous system in the Cœlenterata and Echinodermata) consisted in the conversion of the inner ends of the epithelial cells into nerve-fibres; that is to say, that the first differentiation resulted in the formation of a layer of white matter on the inner side of the epidermis. The next stage was the separation of a deeper layer of the epidermis as a layer of ganglion cells from the superficial epithelial layer,i.e.the formation of a middle layer of ganglion cells and an outer epithelial layer. Thus, phylogenetically, the same three layers as those which first make their appearance in the ontogeny of the vertebrate nervous system became successively differentiated, and in both cases they are clearly placed in the same positions, because the central canal of the vertebrate nervous system, as formed by an involution, is at the true outer surface, and the external part of the cord is at the true inner surface.
It is probable that a very sharp distinction between the white and grey matter is a feature acquired in the higher Vertebrata, since in Amphioxus there is no such sharp separation; though the nerve-fibres are mainly situated externally and the nerve-cells internally.
As already stated in Chapter XII. the primitive division of the nervous axis was probably not into brain and spinal cord, but into (1) a fore-brain, representing the ganglion of the præoral lobe, and (2) the posterior part of the nervous axis, consisting of the mid- and hind-brains and the spinal cord. This view of the division of the central nervous system fits in fairly satisfactorily with the facts of development. The fore-brain is, histologically, more distinct from the posterior part of the nervous system than the posterior parts are from each other; the front end of the notochord forms the boundary between these two parts of the central nervous system (videfig. 253), ending as it does at the front termination of the floor of the mid-brain, and finally,the nerves of the fore-brain have a different character to those of the mid- and hind-brain.
This primitive division of the central nervous system is lost in all the true Vertebrata, and in its place there is a secondary division—corresponding with the secondary vertebrate head—into a brain and spinal cord. The brain, as it is established in these forms, is again divided into a fore-brain, a mid-brain and a hind-brain. The fore-brain is, as we have already seen, the original ganglion of the præoral lobe. The mid-brain appears to be the lobe, or ganglion, of the third pair of nerves (first pair of segmental nerves), while the hind-brain is a more complex structure, each section of which (perhaps indicated by the constrictions which often appear at an early stage of development) giving rise to a pair of segmental nerves is, roughly speaking, homologous with the whole mid-brain.
The type of differentiation of each of the primitively simple vesicles forming the fore-, the mid- and the hind-brains is very uniform throughout the Vertebrate series, but it is highly instructive to notice the great variations in the relative importance of the parts of the brain in the different types. This is especially striking in the case of the fore-brain, where the cerebral hemispheres, which on embryological grounds we may conclude to have been hardly differentiated as distinct parts of the fore-brain in the most primitive types now extinct, gradually become more and more prominent, till in the highest Mammalia they constitute a more important section of the brain than the whole of the remaining parts put together.
The little that is known with reference to the significance of the more or less corresponding outgrowths of the floor and roof of the thalamencephalon, constituting the infundibulum and pineal gland, has already been mentioned in connection with the development of these parts.
Bibliography.
(332)C. J. Carus.Versuch einer Darstellung d. Nervensystems, etc.Leipzig, 1814.(333)J. L. Clark. “Researches on the development of the spinal cord in Man, Mammalia and Birds.”Phil. Trans., 1862.(334)E. Dursy. “Beiträge zur Entwicklungsgeschichte des Hirnanhanges.”Centralblatt f. d. med. Wissenschaften, 1868.Nr.8.(335)E. Dursy.Zur Entwicklungsgeschichte des Kopfes des Menschen und der höheren Wirbelthiere.Tübingen, 1869.(336)A. Ecker. “Zur Entwicklungsgeschichte der Furchen und Windungen der Grosshirn-Hemisphären im Foetus des Menschen.”Archiv f. Anthropologie, v. Ecker und Lindenschmidt.Vol.III.1868.(337)E. Ehlers. “Die Epiphyse am Gehirn d. Plagiostomen.”Zeit. f. wiss. Zool.Vol.XXX., suppl.1878.(338)P. Flechsig.Die Leitungsbahnen im Gehirn und Rückenmark des Menschen. Auf Grund entwicklungsgeschichtlicher Untersuchungen.Leipzig, 1876.(339)V. Hensen. “Zur Entwicklung des Nervensystems.”Virchow’s Archiv,Bd.XXX. 1864.(340)L. Löwe. “Beiträge z. Anat. u. z. Entwick. d. Nervensystems d. Säugethiere u. d. Menschen.” Berlin, 1880.(341)L. Löwe. “Beiträge z. vergleich. Morphogenesis d. centralen Nervensystems d. Wirbelthiere.”Mittheil. a. d. embryol. Instit. Wien,Vol.II. 1880.(342)A. M. Marshall. “The Morphology of the Vertebrate Olfactory organ.”Quart. J. of Micr. Science,Vol.XIX. 1879.(343)V. v. Mihalkovics.Entwicklungsgeschichte d. Gehirns. Leipzig, 1877.(344)W. Müller. “Ueber Entwicklung und Bau der Hypophysis und des Processus infundibuli cerebri.”Jenaische Zeitschrift.Bd.VI. 1871.(345)H. Rahl-Rückhard. “Die gegenseitigen Verhältnisse d. Chorda, Hypophysis etc. bei Haifischembryonen, nebst Bemerkungen üb. d. Deutung d. einzelnen Theile d. Fischgehirns.”Morphol. Jahrbuch,Vol.VI.1880.(346)H. Rathke. “Ueber die Entstehung der glandula pituitaria.”Müller’s Archiv f. Anat. und Physiol.,Bd.V. 1838.(347)C. B. Reichert.Der Bau des menschlichen Gehirns.Leipzig, 1859 u. 1861.(348)F. Schmidt. “Beiträge zur Entwicklungsgeschichte des Gehirns.”Zeitschrift f. wiss. Zoologie, 1862.Bd.XI.(349)G. Schwalbe. “Beitrag z. Entwick. d. Zwischenhirns.”Sitz. d. Jenaischen Gesell. f. Med. u. Naturwiss.Jan. 23, 1880.(350)Fried. Tiedemann.Anatomie und Bildungsgeschichte des Gehirns im Foetus des Menschen.Nürnberg, 1816.
The development of the Cranial and Spinal Nerves[169].
All the nerves are outgrowths of the central nervous system, but the differences in development between the cranial and spinal nerves are sufficiently great to make it convenient to treat them separately.
Spinal nerves. The posterior roots of the spinal nerves, as well as certain of the cranial nerves, arise in the same manner, and from the same structure, and are formed considerably before the anterior roots. Elasmobranch fishes may be taken as the type to illustrate the mode of formation of the spinal nerves.
The whole of the nerves in question arise as outgrowths of a median ridge of cells, which makes its appearance on the dorsal side of the spinal cord (fig. 264 A,pr). This ridge has been called by Marshall the neural crest. At each point, where a pair of nerves will be formed, two pear-shaped outgrowths project from it, one on each side; and apply themselves closely to the walls of the spinal cord (fig. 264 B,pr). These outgrowths are the rudiments of the posterior nerves. While still remaining attached to the dorsal summit of the neural cord they grow to a considerable size (fig. 264 B,pr).
Illustration: Figure 264aFig. 264 A. Transverse section through a pristiurus embryo shewing the proliferation of cells to form the neural crest.pr.neural crest;nc.neural canal;ch.notochord;ao.aorta.
Fig. 264 A. Transverse section through a pristiurus embryo shewing the proliferation of cells to form the neural crest.pr.neural crest;nc.neural canal;ch.notochord;ao.aorta.
Illustration: Figure 264bFig. 264 B. Transverse section through the trunk of an embryo slightly older than fig. 28 E.nc.neural canal;pr.posterior root of spinal nerve;x.subnotochordal rod;ao.aorta;sc.somatic mesoblast;sp.splanchnic mesoblast;mp.muscle-plate;mp´.portion of muscle-plate converted into muscle;Vv.portion of the vertebral plate which will give rise to the vertebral bodies;al.alimentary tract.
Fig. 264 B. Transverse section through the trunk of an embryo slightly older than fig. 28 E.nc.neural canal;pr.posterior root of spinal nerve;x.subnotochordal rod;ao.aorta;sc.somatic mesoblast;sp.splanchnic mesoblast;mp.muscle-plate;mp´.portion of muscle-plate converted into muscle;Vv.portion of the vertebral plate which will give rise to the vertebral bodies;al.alimentary tract.
Illustration: Figure 265Fig. 265. Vertical longitudinal section through part of the trunk of a young Scyllium embryo.com.commissure uniting the dorsal ends of the posterior nerve-roots;pr.ganglia of posterior roots;ar.anterior roots;st.segmental tubes;sd.segmental duct;g.e.epithelium lining the body cavity in the region of the future germinal ridge.
Fig. 265. Vertical longitudinal section through part of the trunk of a young Scyllium embryo.com.commissure uniting the dorsal ends of the posterior nerve-roots;pr.ganglia of posterior roots;ar.anterior roots;st.segmental tubes;sd.segmental duct;g.e.epithelium lining the body cavity in the region of the future germinal ridge.
The attachment to the dorsal summit is not permanent, butbefore describing the further fate of the nerve-rudiments it is necessary to say a few words as to the neural crest. At the period when the nerves have begun to shift their attachment to the spinal cord, there makes its appearance, in Elasmobranchii, a longitudinal commissure connecting the dorsal ends of all the spinal nerves (figs.265,266com), as well as those of the vagus and glossopharyngeal nerves. This commissure has as yet only been found in a complete form in Elasmobranchii;but it is nevertheless to be regarded as a very important morphological structure.
Illustration: Figure 266Fig. 266. Spinal Nerves of Scyllium in longitudinal section to shew the commissure connecting them.A. Section through a series of nerves.B. Highly magnified view of the dorsal part of a single nerve, and of the commissure connected with it.com.commissure;sp.g.ganglion of posterior root;ar.anterior root.
Fig. 266. Spinal Nerves of Scyllium in longitudinal section to shew the commissure connecting them.A. Section through a series of nerves.B. Highly magnified view of the dorsal part of a single nerve, and of the commissure connected with it.com.commissure;sp.g.ganglion of posterior root;ar.anterior root.
It is probable, though the point has not yet been definitely made out, that this commissure is derived from the neural crest, which appears therefore to separate into two cords, one connected with each set of dorsal roots.
Illustration: Figure 267Fig. 267. Section through the dorsal part of the trunk of a Torpedo embryo.pr.posterior root of spinal nerve;g.spinal ganglion;n.nerve;ar.anterior root of spinal nerve;ch.notochord;nc.neural canal;mp.muscle-plate.
Fig. 267. Section through the dorsal part of the trunk of a Torpedo embryo.pr.posterior root of spinal nerve;g.spinal ganglion;n.nerve;ar.anterior root of spinal nerve;ch.notochord;nc.neural canal;mp.muscle-plate.
Returning to the original attachment of the nerve-rudiments to the medullary wall, it has been already stated that this attachment is not permanent. It becomes, in fact, at about the time of the appearance of the above commissure, either extremely delicate or absolutely interrupted.
The nerve-rudiment now becomes divided into three parts (figs.267and268), (1) a proximal rounded portion, to which is attached the longitudinal commissure (pr); (2) an enlarged portion, forming the rudiment of a ganglion (gandsp g); (3) a distal portion, forming the commencement of the nerve (n). The proximal portion may very soon be observed to be united with the side of the spinal cord at a very considerable distance from its original point of attachment. Moreover the proximal portion of the nerve is attached, not by its extremity, but by its side, to the spinal cord (fig. 268x). The dorsal extremities of the posterior roots are therefore free.
This attachment of the posterior nerve-root to the spinal cord is, on account of its small size, very difficult to observe. In favourable specimens there may however be seen a distinct cellular prominence from the spinal cord, which becomes continuous with a small prominence on the lateral border of the nerve-root near its proximal extremity. The proximal extremity of the nerve is composed of cells, which, by their small size and circular form, are easily distinguished from those which form the succeeding or ganglionic portion of the nerve. This part has a swollen configuration, and is composed of large elongated cells with oval nuclei. The remainder of the rudiment forms the commencement of the true nerve. This also is, at first, composed of elongated cells[170].
It is extremely difficult to decide whether the permanent attachment of the posterior nerve-roots to the spinal cord is entirely a new formation, or merely due to the shifting of the original point of attachment. I am inclined to adopt the former view, which is also held by Marshall and His, but may refer tofig. 269, shewing the roots after they have become attached to the side, as distinct evidence in favour of the view that the attachment simply becomes shifted, a process which might perhaps be explained by a growth of the dorsal part of the spinal cord. The change of position in the case of some of the cranial nerves is, however, so great that I do not think that it is possible to account for it without admitting the formation of a new attachment.
Illustration: Figure 268Fig. 268. Section through the dorsal region of a Pristiurus embryo.pr.posterior root;sp.g.spinal ganglion;n.nerve;x.attachment of ganglion to spinal cord;nc.neural canal;mp.muscle-plate;ch.notochord;i.investment of spinal cord.
Fig. 268. Section through the dorsal region of a Pristiurus embryo.pr.posterior root;sp.g.spinal ganglion;n.nerve;x.attachment of ganglion to spinal cord;nc.neural canal;mp.muscle-plate;ch.notochord;i.investment of spinal cord.
The anterior roots of the spinal nerves appear somewhat later than the posterior roots, but while the latter are still quite small. Each of them (fig. 269ar) arises as a small but distinct conical outgrowth from a ventral corner of the spinal cord, before the latter has acquired its covering of white matter. From the very first the rudiments of the anterior roots have a somewhat fibrous appearance and an indistinct form of peripheral termination, while the protoplasm of which they are composed becomes attenuated towards its end. They differ from the posterior roots in never shifting their point of attachment to the spinal cord, in not being united with each other by a commissure, and in never developing a ganglion.
The anterior roots grow rapidly, and soon form elongated cords of spindle-shaped cells with wide attachments to the spinal cord (fig. 267). At first they pass obliquely and nearly horizontally outwards, but, before reaching the muscle-plates, they take a bend downwards.
One feature of some interest with reference to the anterior roots is the fact that they arise not vertically below, but alternately with the posterior roots: a condition which persists in the adult. They are at first quite separate from the posterior roots; but about the stage represented infig. 267a junction is effected between each posterior root and the corresponding anterior root. The anterior root joins the posterior at some little distance below its ganglion (figs.265and266).