Illustration: Figure 353Fig. 353. Transverse sections through a Chick embryo with twenty-one mesoblastic somites to shew the formation of the pericardial cavity, A. being the anterior section.p.p.body cavity;p.c.pericardial cavity;al.alimentary cavity;au.auricle;v.ventricle;s.v.sinus venosus;d.c.ductus Cuvieri;ao.aorta;mp.muscle-plate;mc.medullary cord.
Fig. 353. Transverse sections through a Chick embryo with twenty-one mesoblastic somites to shew the formation of the pericardial cavity, A. being the anterior section.p.p.body cavity;p.c.pericardial cavity;al.alimentary cavity;au.auricle;v.ventricle;s.v.sinus venosus;d.c.ductus Cuvieri;ao.aorta;mp.muscle-plate;mc.medullary cord.
With the complete separation of the pericardial cavity from the body cavity, the first period in the development of these parts is completed, and the relations of the body cavity to thepericardial cavity become precisely those found in the embryos of Elasmobranchii. The later changes are however very different. Whereas in Fishes the right and left sections of the body cavity dorsal to the pericardial cavity soon atrophy, in the higher types, in correlation with the relatively backward situation of the heart, they rapidly become larger, and receive the lungs which soon sprout out from the throat.
The diverticula which form the lungs grow out into the splanchnic mesoblast, in front of the body cavity; but as they grow, they extend into the two anterior compartments of the body cavity, each attached by its mesentery to the mesentery of the gut (fig. 354,lg). They soon moreover extend beyond the region of the pericardium into the undivided body cavity behind. This holds not only for the embryos of the Amphibia and Sauropsida, but also for those of Mammalia.
Illustration: Figure 354Fig. 354. Section through the cardiac region of an embryo of Lacerta Muralis of 9mm.to shew the mode of formation of the pericardial cavity.ht.heart;pc.pericardial cavity;al.alimentary tract;lg.lung;l.liver;pp.body cavity;md.open end of Müllerian duct;wd.Wolffian duct;vc.vena cava inferior;ao.aorta;ch.notochord;mc.medullary cord.
Fig. 354. Section through the cardiac region of an embryo of Lacerta Muralis of 9mm.to shew the mode of formation of the pericardial cavity.ht.heart;pc.pericardial cavity;al.alimentary tract;lg.lung;l.liver;pp.body cavity;md.open end of Müllerian duct;wd.Wolffian duct;vc.vena cava inferior;ao.aorta;ch.notochord;mc.medullary cord.
To understand the further changes in the pericardial cavity it is necessary to bear in mind its relations to the adjoining parts. It lies at this period completely ventral to the two anterior prolongations of the body cavity containing the lungs (fig. 354). Its dorsal wall is attached to the gut, and is continuous with the mesentery of the gut passing to the dorsal abdominal wall, forming the posterior mediastinum of human anatomy.
The changes which next ensue consist essentially in the enlargement of the sections of the body cavity dorsal to the pericardial cavity. This enlargement takes place partly by the elongation of the posterior mediastinum, but still more by the two divisions of the body cavity which contain the lungs extending themselves ventrally round the outside of the pericardialcavity. This process is illustrated byfig. 355, taken from an embryo Rabbit. The two dorsal sections of the body cavity (pl.p) finally extend so as completely to envelope the pericardial cavity (pc), remaining however separated from each other below by a lamina extending from the ventral wall of the pericardial cavity to the body wall, which forms the anterior mediastinum of human anatomy.
Illustration: Figure 355Fig. 355. Section through an advanced embryo of a Rabbit to shew how the pericardial cavity becomes surrounded by the pleural cavities.ht.heart;pc.pericardial cavity;pl.ppleural cavity;lg.lung;al.alimentary tract;ao.dorsal aorta;ch.notochord;rp.rib;st.sternum;sp.c.spinal cord.
Fig. 355. Section through an advanced embryo of a Rabbit to shew how the pericardial cavity becomes surrounded by the pleural cavities.ht.heart;pc.pericardial cavity;pl.ppleural cavity;lg.lung;al.alimentary tract;ao.dorsal aorta;ch.notochord;rp.rib;st.sternum;sp.c.spinal cord.
By these changes the pericardial cavity is converted into a closed bag, completely surrounded at its sides by the two lateral halves of the body cavity, which were primitively placed dorsally to it. These two sections of the body cavity, which in Amphibia and Sauropsida remain in free communication with the undivided peritoneal cavity behind, may, from the fact of their containing the lungs, be called thepleural cavities.
In Mammalia a further change takes place, in that, by the formation of a vertical partition across the body cavity, known as thediaphragm, the pleural cavities, containing the lungs,become isolated from the remainder of the body or peritoneal cavity. As shewn by their development the so-called pleuræ or pleural sacks are simply the peritoneal linings of the anterior divisions of the body cavity, shut off from the remainder of the body cavity by the diaphragm.
The exact mode of formation of the diaphragm is not fully made out; the account of it recently given by Cadiat (No.491) not being in my opinion completely satisfactory.
Bibliography.
(491)M. Cadiat. “Du développement de la partie céphalothoracique de l'embryon, de la formation du diaphragme, des pleures, du péricarde, du pharynx et de l'Å“sophage.â€Journal de l'Anatomie et de la Physiologie,Vol.XIV. 1878.
Vascular System.
The actual observations bearing on the origin of the vascular system, using the term to include the lymphatic system, are very scanty. It seems probable, mainly it must be admitted onà priorigrounds, that vascular and lymphatic systems have originated from the conversion of indefinite spaces, primitively situated in the general connective tissue, into definite channels. It is quite certain that vascular systems have arisen independently in many types; a very striking case of the kind being the development in certain parasitic Copepoda of a closed system of vessels with a red non-corpusculated blood (E. van Beneden, Heider), not found in any other Crustacea. Parts of vascular systems appear to have arisen in some cases by a canalization of cells.
The blood systems may either be closed or communicate with the body cavity. In cases where the primitive body cavity is atrophied or partially broken up into separate compartments (Insecta, Mollusca, Discophora, etc.) a free communication between the vascular system and the body cavity is usually present; but in these cases the communication is no doubt secondary. On the whole it would seem probable that the vascular system has in most instances arisen independently of the body cavity, at least in types where the body cavity ispresent in a well-developed condition. As pointed out by the Hertwigs, a vascular system is always absent where there is not a considerable development of connective tissue.
As to the ontogeny of the vascular channels there is still much to be made out both in Vertebrates and Invertebrates.
The smaller channels often rise by a canalization of cells. This process has been satisfactorily studied by Lankester in the Leech[221], and may easily be observed in the blastoderm of the Chick or in the epiploon of a newly born Rabbit (Schäfer, Ranvier). In either case the vessels arise from a network of cells, the superficial protoplasm and part of the nuclei giving rise to the walls, and the blood-corpuscles being derived either from nucleated masses set free within the vessels (the Chick) or from blood-corpuscles directly differentiated in the axes of the cells (Mammals).
Larger vessels would seem to be formed from solid cords of cells, the central cells becoming converted into the corpuscles, and the peripheral cells constituting the walls. This mode of formation has been observed by myself in the case of the Spider’s heart, and by other observers in other Invertebrata. In the Vertebrata a more or less similar mode of formation appears to hold good for the larger vessels, but further investigations are still required on this subject. Götte finds that in the Frog the larger vessels are formed as longitudinal spaces, and that the walls are derived from the indifferent cells bounding these spaces, which become flattened and united into a continuous layer.
The early formation of vessels in the Vertebrata takes place in the splanchnic mesoblast; but this appears due to the fact that the circulation is at first mainly confined to the vitelline region, which is covered by splanchnic mesoblast.
The Heart.
Illustration: Figure 356Fig. 356. Section through the developing heart of an embryo of an Elasmobranch(Pristiurus).al.alimentary tract;sp.splanchnic mesoblast;so.somatic mesoblast;ht.heart.
Fig. 356. Section through the developing heart of an embryo of an Elasmobranch(Pristiurus).al.alimentary tract;sp.splanchnic mesoblast;so.somatic mesoblast;ht.heart.
The heart is essentially formed as a tubular cavity in the splanchnic mesoblast, on the ventral side of the throat, immediately behind the region of the visceral clefts. The walls of this cavity are formed of two layers, an outer thicker layer, which has at first only the form of a half tube, being incomplete on its dorsal side; and an inner lamina formed of delicate flattened cells. The latter is the epithelioid lining of the heart, and the cavity it contains the true cavity of the heart. The outer layer gives rise to the muscular wall and peritoneal covering of the heart. Though at first it has only the form of a half tube (fig.356), it soon becomes folded in on the dorsal side so as to form for the heart a complete muscular wall. Its two sides, after thus meeting to complete the tube of the heart, remain at first continuous with the splanchnic mesoblast surrounding the throat, and form a provisional mesentery—the mesocardium—which attaches the heart to the ventral wall of the throat. The superficial stratum of the wall of the heart differentiates itself as the peritoneal covering. The inner epithelioid tube takes its origin at the time when the general cavity of the heart is being formed by the separation of the splanchnic mesoblast from the hypoblast. During this process (fig. 357) a layer of mesoblast remains close to the hypoblast, but connected with the main massof the mesoblast by protoplasmic processes. A second layer next becomes split from the splanchnic mesoblast, connected with the first layer by the above-mentioned protoplasmic processes. These two layers form together the epithelioid lining of the heart; between them is the cavity of the heart, which soon loses the protoplasmic trabeculæ which at first traverse it. The cavity of the heart may thus be described as being formed by a hollowing out of the splanchnic mesoblast, and resembles in its mode of origin that of other large vascular trunks.
Illustration: Figure 357Fig. 357. 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. 357. 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.
Illustration: Figure 358Fig. 358. Transverse section through the head of a Rabbit of the same age as fig. 144 B.(From Kölliker.)B is a more highly magnified representation of part of A.rf.medullary groove;mp.medullary plate;rw.medullary fold;h.epiblast;dd.hypoblast;dd´.notochordal thickening of hypoblast;sp.undivided mesoblast;hp.somatic mesoblast;dfp.splanchnic mesoblast;ph.pericardial section of body cavity;ahh.muscular wall of heart;ihh.epithelioid layer of heart;mes.lateral undivided mesoblast;sw.part of the hypoblast which will form the ventral wall of the pharynx.
Fig. 358. Transverse section through the head of a Rabbit of the same age as fig. 144 B.(From Kölliker.)B is a more highly magnified representation of part of A.rf.medullary groove;mp.medullary plate;rw.medullary fold;h.epiblast;dd.hypoblast;dd´.notochordal thickening of hypoblast;sp.undivided mesoblast;hp.somatic mesoblast;dfp.splanchnic mesoblast;ph.pericardial section of body cavity;ahh.muscular wall of heart;ihh.epithelioid layer of heart;mes.lateral undivided mesoblast;sw.part of the hypoblast which will form the ventral wall of the pharynx.
The above description applies only to the development of the heart in those types in which it is formed at a periodafterthe throat has become a closed tube (Elasmobranchii, Amphibia, Cyclostomata, Ganoids (?)). In a number of other cases, in which the heart is formed before the conversion of the throat into a closed tube, of which the most notable is that of Mammals (Hensen, Götte, Kölliker), the heart arises as two independenttubes (fig. 358), which eventually coalesce into an unpaired structure.
In Mammals the two tubes out of which the heart is formed appear at the sides of the cephalic plates, opposite the region of the mid- and hind-brain (fig. 358). They arise at a time when the lateral folds which form the ventral wall of the throat are only just becoming visible. Each half of the heart originates in the same way as the whole heart in Elasmobranchii, etc.; and the layer of the splanchnic mesoblast, which forms the muscular wall for each part (ahh), has at first the form of a half tube open below to the hypoblast.
Illustration: Figure 359Fig. 359. Two diagrammatic sections through the region of the hind-brain of an embryo Chick of about 36 hours illustrating the formation of the heart.hb.hind-brain;nc.notochord;E.epiblast;so.somatopleure;sp.splanchnopleure;d.alimentary tract;hy.hypoblast;hz.heart;of.vitelline veins.
Fig. 359. Two diagrammatic sections through the region of the hind-brain of an embryo Chick of about 36 hours illustrating the formation of the heart.hb.hind-brain;nc.notochord;E.epiblast;so.somatopleure;sp.splanchnopleure;d.alimentary tract;hy.hypoblast;hz.heart;of.vitelline veins.
On the formation of the lateral folds of the splanchnic walls, the two halves of the heart become carried inwards and downwards, and eventuallymeet on the ventral side of the throat. For a short time they here remain distinct, but soon coalesce into a single tube.
In Birds, as in Mammals, the heart makes its appearance as two tubes, but arises at a period when the formation of the throat is very much more advanced than in the case of Mammals. The heart arises immediately behind the point up to which the ventral wall of the throat is established and thus has at first a Lambda-shaped form. At the apex of the Lambda, which forms the anterior end of the heart, the two halves are in contact (fig. 357), though they have not coalesced; while behind they diverge to be continued as the vitelline veins. As the folding in of the throat is continued backwards the two limbs of the heart are brought together and soon coalesce from before backwards into a single structure.Fig. 359A and B shews the heart during this process. The two halves have coalesced anteriorly (A) but are still widely separated behind (B). In Teleostei the heart is formed as in Birds and Mammals by the coalescence of two tubes, and it arises before the formation of the throat.
The fact that the heart arises in so many instances as a double tube might lead to the supposition that the ancestral Vertebrate had two tubes in the place of the present unpaired heart.
The following considerations appear to me to prove that this conclusion cannot be accepted. If the folding in of the splanchnopleure to form the throat were deferred relatively to the formation of the heart, it is clear that a modification in the development of the heart would occur, in that the two halves of the heart would necessarily be formed widely apart, and only eventually united on the folding in of the wall of the throat. It is therefore possible to explain the double formation of the heart without having recourse to the above hypothesis of an ancestral Vertebrate with two hearts. If the explanation just suggested is the true one the heart should only be formed as two tubes when it arises prior to the formation of the throat, and as a single tube when formed after the formation of the throat. Since this is invariably found to be so, it may be safely concludedthat the formation of the heart as two cavities is a secondary mode of development, which has been brought about by variations in the period of the closing in of the wall of the throat.
The heart arises continuously with the sinus venosus, which in the Amniotic Vertebrata is directly continued into the vitelline veins. Though at first it ends blindly in front, it is very soon connected with the foremost aortic arches.
The simple tubular heart, connected as above described, grows more rapidly than the chamber in which it is contained, and is soon doubled upon itself, acquiring in this way an S-shaped curvature, the posterior portion being placed dorsally, and the anterior ventrally. A constriction soon appears between the dorsal and ventral portions.
The dorsal section becomes partially divided off behind from the sinus venosus, and constitutes the relatively thin-walledauricularsection of the heart; while the ventral portion, after becoming distinct anteriorly from a portion continued forwards from it to the origin of the branchial arteries, which may be called thetruncus arteriosus, acquires very thick spongy muscular walls, and becomes theventriculardivision of the heart.
The further changes in the heart are but slight in the case of the Pisces. A pair of simple membranous valves becomes established at the auriculo-ventricular orifice, and further changes take place in the truncus arteriosus. This part becomes divided in Elasmobranchii, Ganoidei, and Dipnoi into a posterior section, called theconus arteriosus, provided with a series of transverse rows of valves, and an anterior section, called thebulbus arteriosus, not provided with valves, and leading into the branchial arteries. In most Teleostei (except Butirinus and a few other forms) the conus arteriosus is all but obliterated, and the anterior row of its valves alone preserved; and the bulbus is very much enlarged[222].
In the Dipnoi important changes in the heart are effected, as compared with other Fishes, by the development of true lungs. Both the auricular and ventricular chamber may be imperfectly divided into two, and in the conus a partial longitudinal septum is developed in connection with a longitudinal row of valves[223].
In Amphibia the heart is in many respects similar to that of the Dipnoi. Its curvature is rather that of a screw than of a simple S. The truncus arteriosus lies to the left, and is continued into the ventricle which lies ventrally and more to the right, and this again into the dorsally placed auricular section.
After the heart has reached the piscine stage, the auricular section (Bombinator) becomes prolonged into a right and left auricular appendage. A septum next grows from the roof of the auricular portion of the heartobliquely backwards and towards the left, and divides it in two chambers; the right one of which remains continuous with the sinus venosus, while the left one is completely shut off from the sinus, though it soon enters into communication with the newly established pulmonary veins. The truncus arteriosus[224]is divided into a posteriorconus arteriosus(pylangium) and an anteriorbulbus(synangium). The former is provided with a proximal row of valves at its ventricular end, and a distal row at its anterior end near the bulbus. It is also provided with a longitudinal septum, which is no doubt homologous with the septum in the conus arteriosus of the Dipnoi. The bulbus is well developed in many Urodela, but hardly exists in the Anura.
In the Amniota further changes take place in the heart, resulting in the abortion of the distal rows of valves of the conus arteriosus[225], and in the splitting up of the whole truncus arteriosus into three vessels in Reptilia, and two in Birds and Mammals, each opening into the ventricular section of the heart, and provided with a special set of valves at its commencement. In Birds and Mammals the ventricle becomes moreover completely divided into two chambers, each communicating with one of the divisions of the primitive truncus, known in the higher types as the systemic and pulmonary aortæ. The character of the development of the heart in the Amniota will be best understood from a description of what takes place in the Chick.
In Birds the originally straight heart (fig. 109) soon becomes doubled up upon itself. The ventricular portion becomes placed on the ventral and right side, while the auricular section is dorsal and to the left. The two parts are separated from each other by a slight constriction known as the canalis auricularis. Anteriorly the ventricular cavity is continued into the truncus, and the venous or auricular portion of the heart is similarly connected behind with the sinus venosus. The auricular appendages grow out from the auricle at a very early period. The general appearance of the heart, as seen from the ventral side on the fourth day, is shewn infig. 360. Although the external divisions of the heart are well marked even before this stage, it is not till the end of the third day that the internal partitions become apparent; and, contrary to what might have been anticipated from the evolution of these parts in the lower types, the ventricular septum is the first to be established.
It commences on the third day as a crescentic ridge or fold springing from the convex or ventral side of the rounded ventricular portion of the heart, and on the fourth day grows rapidly across the ventricular cavity towards the concave or dorsal side. It thus forms an incomplete longitudinal partition, extending from the canalis auricularis to the commencement of the truncus arteriosus, and dividing the twisted ventricular tube into two somewhat curved canals, one more to the left and above, the other to the right and below. These communicate with each other, above the free edge of the partition, along its whole length.
Illustration: Figure 360Fig. 360. Heart of a Chick on the fourth day of incubation viewed from the ventral surface.l.a.left auricular appendage;C.A.canalis auricularis;v.ventricle;b.truncus arteriosus.
Fig. 360. Heart of a Chick on the fourth day of incubation viewed from the ventral surface.l.a.left auricular appendage;C.A.canalis auricularis;v.ventricle;b.truncus arteriosus.
Externally the ventricular portion as yet shews no division into two parts.
By the fifth day the venous end of the heart, though still lying somewhat to the left and above, is placed as far forwards as the arterial end, the whole organ appearing to be drawn together. The ventricular septum is complete.
The apex of the ventricles becomes more and more pointed. In the auricular portion a small longitudinal fold appears as the rudiment of the auricular septum, while in the canalis auricularis, which is now at its greatest length, there is also to be seen a commencement of the valvular structures tending to separate the cavity of the auricles from those of the ventricles.
About the 106th hour, a septum begins to make its appearance in the truncus arteriosus in the form of a longitudinal fold, which according to Tonge (No.495) starts at the end of the truncus furthest removed from the heart. It takes origin from the wall of the truncus between the fourth and fifth pairs of arches, and grows downwards in such a manner as to divide the truncus into two channels, one of which leads from the heart to the third and fourth pairs of arches, and the other to the fifth pair. Its course downwards is not straight but spiral, and thus the two channels into which it divides the truncus arteriosus wind spirally the one round the other.
At the time when the septum is first formed, the opening of the truncus arteriosus into the ventricles is narrow or slit-like, apparently in order to prevent the flow of the blood back into the heart. Soon after the appearance of the septum, however, semilunar valves (Tonge,No.495) are developed from the wall of that portion of the truncus which lies between the free edge of the septum and the cavity of the ventricles[226].
The ventral and the dorsal pairs of valves are the first to appear: the former as two small solid prominences separated from each other by a narrow groove; the latter as a single ridge, in the centre of which is a prominence indicating the point where the ridge will subsequently become divided into two. The outer valves appear opposite each other, at a considerably later period.
Illustration: Figure 361Fig. 361. Two views of the heart of a Chick upon the fifth day of incubation.A. from the ventral, B. from the dorsal side.l.a.left auricular appendage;r.a.right auricular appendage;r.v.right ventricle;l.v.left ventricle;b.truncus arteriosus.
Fig. 361. Two views of the heart of a Chick upon the fifth day of incubation.A. from the ventral, B. from the dorsal side.l.a.left auricular appendage;r.a.right auricular appendage;r.v.right ventricle;l.v.left ventricle;b.truncus arteriosus.
As the septum grows downwards towards the heart, it finally reaches the position of these valves. One of its edges then passes between the two ventral valves, and the other unites with the prominence on the dorsal valve-ridge. At the same time the growth of all the parts causes the valves to appear to approach the heart, and thus to be placed quite at the top of the ventricular cavities. The free edge of the septum of the truncus now fuses with the ventricular septum, and thus the division of the truncus into two separate channels, each provided with three valves, and each communicating with a separate side of the heart, is complete; the position of the valves not being very different from that in the adult heart.
That division of the truncus which opens into the fifth pair of arches is the one which communicates with the right ventricle, while that which opens into the third and fourth pairs communicates with the left ventricle. The former becomes the pulmonary artery, the latter the commencement of the systemic aorta.
The external constriction actually dividing the truncus into two vessels does not begin to appear till the septum has extended some way back towards the heart.
The semilunar valves become pocketed at a period considerably later than their first formation (from the 147th to the 165th hour) in the order of their appearance.
At the end of the sixth day, and even on the fifth day (figs.361and362), the appearance of the heart itself, without reference to the vessels which come from it, is not very dissimilar from that of the adult. The originalprotuberance to the right now forms the apex of the ventricles, and the two auricular appendages are placed at the anterior extremity of the heart. The most noticeable difference (in the ventral view) is the still externally undivided condition of the truncus arteriosus.
Illustration: Figure 362Fig. 362. Heart of a Chick upon the sixth day of incubation, from the ventral surface.l.a.left auricular appendage;r.a.right auricular appendage;r.v.right ventricle;l.v.left ventricle;b.truncus arteriosus.
Fig. 362. Heart of a Chick upon the sixth day of incubation, from the ventral surface.l.a.left auricular appendage;r.a.right auricular appendage;r.v.right ventricle;l.v.left ventricle;b.truncus arteriosus.
The subsequent changes which the heart undergoes are concerned more with its internal structure than with its external shape. Indeed, during the next three days,viz.the eighth, ninth, and tenth, the external form of the heart remains nearly unaltered.
In the auricular portion, however, the septum which commenced on the fifth day becomes now more conspicuous. It is placed vertically, and arises from the ventral wall; commencing at the canalis auricularis and proceeding towards the opening into the sinus venosus.
This latter structure gradually becomes reduced so as to become a special appendage of the right auricle. The inferior vena cava enters the sinus obliquely from the right, so that its blood has a tendency to flow towards the left auricle of the heart, which is at this time the larger of the two.
The valves between the ventricles and auricles are now well developed, and it is about this time that the division of the truncus arteriosus into the aorta and pulmonary artery becomes visible from the exterior.
By the eleventh to the thirteenth day the right auricle has become as large as the left, and the auricular septum much more complete, though there is still a small opening, theforamen ovale, by which the two cavities communicate with each other.
The most important feature in which the development of the Reptilian heart differs from that of Birds is the division of the truncus into three vessels, instead of two. The three vessels remain bound up in a common sheath, and appear externally as a single trunk. The vessel not represented in Birds is that which is continued into the left aortic arch.
In Mammals the early stages in the development of the heart present no important points of difference from those of Aves. The septa in the truncus, in the ventricular, and in the auricular cavities are formed, so far as is known, in the same way and at the same relative periods in both groups. In the embryo Man, the Rabbit, and other Mammals the division of the ventricles is made apparent externally by a deep cleft, which, though evanescent in these forms, is permanent in the Dugong.
The attachment of the auriculo-ventricular valves to the wall of the ventricle, and the similar attachment of the left auriculo-ventricular valves in Birds, have been especially studied by Gegenbaur and Bernays (No.492),and deserve to be noticed. In the primitive state the ventricular walls have throughout a spongy character; and the auriculo-ventricular valves are simple membranous projections like the auriculo-ventricular valves of Fishes. Soon however the spongy muscular tissue of both the ventricular and auricular walls, which at first pass uninterruptedly the one into the other, grows into the bases of the valves, which thus become in the main muscular projections of the walls of the heart. As the wall of the ventricle thickens, the muscular trabeculæ, connected at one end with the valves, remain at the other end united with the ventricular wall, and form special bands passing between the two. The valves on the other hand lose their muscular attachment to the auricular walls. This is the condition permanent in Ornithorhynchus. In higher Mammalia the ends of the muscular bands inserted into the valves become fibrous, from the development of intermuscular connective tissue, and the atrophy of the muscular elements. The fibrous parts now form the chordæ tendineæ, and the muscular the musculi papillares.
The sinus venosus in Mammals becomes completely merged into the right auricle, and the systemic division of the truncus arteriosus is apparently not homologous with that in Birds.
In the embryos of all the Craniata the heart is situated very far forwards in the region of the head. This position is retained in Pisces. In Amphibia the heart is moved further back, while in all the Amniota it gradually shifts its position first of all into the region of the neck and finally passes completely within the thoracic cavity. The steps in the change of position may be gathered fromfigs.109,111, and118.
Bibliographyof the Heart.
(492)A. C. Bernays. “Entwicklungsgeschichte d. Atrioventricularklappen.â€Morphol. Jahrbuch,Vol.II. 1876.(493)E. Gasser. “Ueber d. Entstehung d. Herzens beim Hühn.â€Archiv f. mikr. Anat.,Vol.XIV.(494)A. Thomson. “On the development of the vascular system of the fÅ“tus of Vertebrated Animals.â€Edinb. New Phil. Journal,Vol.IX. 1830 and 1831.(495)M. Tonge. “Observations on the development of the semilunar valves of the aorta and pulmonary artery of the heart of the Chick.â€Phil. Trans.CLIX.1869.Videalso Von Baer (291), Rathke (300), Hensen (182), Kölliker (298), Götte (296), and Balfour (292).
Arterial System.
In the embryos of Vertebrata the arterial system consists of a forward continuation of the truncus arteriosus, on the ventralside of the throat (figs.363,abr, and364,a), which, with a few exceptions to be noticed below, divides into as many branches on each side as there are visceral arches. These branches, after traversing the visceral arches, unite on the dorsal side of the throat into a common trunk on each side. This trunk (figs.363and364) after giving off one (or more) vessels to the head (c´andc) turns backwards, and bends in towards the middle line, close to its fellow, immediately below the notochord (figs.21and116) and runs backwards in this situation towards the end of the tail. The two parallel trunks below the notochord fuse very early into a single trunk, the dorsal aorta (figs.363,ad, and364,a´´). There is given off from each collecting trunk from the visceral arches, or from the commencement of the dorsal aorta, a subclavian artery to each of the anterior limbs; from near the anterior end of the dorsal aorta a vitelline artery (or before the dorsal aortæ have united a pair of arteriesfig. 125, RofA and LofA) to the yolk-sack, which subsequently becomes the main visceral artery[227]; and from the dorsal aorta opposite the hind limbs one (or two) arteries on each side—the iliac arteries—to the hind limbs; from these arteries the allantoic arteries are given off in the higher types, which remain as the hypogastric arteries after the disappearance of the allantois.
Illustration: Figure 363Fig. 363. Diagrammatic view of the head of an embryo Teleostean, with the primitive vascular trunks.(From Gegenbaur.)a.auricle;v.ventricle;abr.branchial artery;c´.carotid;ad.dorsal aorta;s.branchial clefts;sv.sinus venosus;dc.ductus Cuvieri;n.nasal pit.
Fig. 363. Diagrammatic view of the head of an embryo Teleostean, with the primitive vascular trunks.(From Gegenbaur.)a.auricle;v.ventricle;abr.branchial artery;c´.carotid;ad.dorsal aorta;s.branchial clefts;sv.sinus venosus;dc.ductus Cuvieri;n.nasal pit.
The primitive arrangement of the arterial trunks is with a few modifications retained in Fishes. With the development of the gills the vessels to the arches become divided into two parts connected by a capillary system in the gill folds,viz.into thebranchial arteries bringing the blood to the gills from the truncus arteriosus, and the branchial veins transporting it to the dorsal aorta. The branchial vessels to those arches which do not bear gills, either wholly or partially atrophy; thus in Elasmobranchii the mandibular trunk, which is fully developed in the embryo (fig. 193, 1av), atrophies, except for a small remnant bringing blood to the rudimentary gill of the spiracle from the branchial vein of the hyoid arch. In Ganoids the mandibular artery atrophies, but the hyoid is usually preserved. In Teleostei both mandibular[228]and hyoid arteries are absent in the adult, except that there is usually left a rudiment of the hyoid, supplying the pseudobranch, which is similar to the rudiment of the mandibular artery in Elasmobranchii. In Dipnoi the mandibular artery atrophies, but the hyoid is sometimes preserved (Protopterus), and sometimes lost.
In Fishes provided with a well developed air-bladder this organ receives arteries, which arise sometimes from the dorsal aorta, sometimes from the cæliac arteries, and sometimes from the dorsal section of the last (fourth) branchial trunk. The latter origin is found in Polypterus and Amia, and seems to have been inherited by the Dipnoi where the air-bladder forms a true lung.
The pulmonary artery of all the air-breathing Vertebrata is derived from the pulmonary artery of the Dipnoi.
In all the types above Fishes considerable changes are effected in the primitive arrangement of the arteries in the visceral arches.
In Amphibia the piscine condition is most nearly retained[229]. The mandibular artery is never developed, and the hyoid artery is imperfect, being only connected with the cephalic vessels and never directly joining the dorsal aorta. It is moreover developed later than the arteries of the true branchial arches behind. The subclavian arteries spring from the common trunks which unite to form the dorsal aorta.
In the Urodela there are developed, in addition to the hyoid,four branchial arteries. The three foremost of these at first supply gills, and in the Perennibranchiate forms continue to do so through life. The fourth does not supply a gill, and very early gives off, as in the Dipnoi, a pulmonary branch.
The hyoid artery soon sends forward a lingual artery from its ventral end, and is at first continued to the carotid which grows forward from the dorsal part of the first branchial vessel.
In the Caducibranchiata, where the gills atrophy, the following changes take place. The remnant of the hyoid is continued entirely into the lingual artery. The first branchial is mainly continued into the carotid and other cephalic branches, but a narrow remnant of the trunk, which originally connected it with the dorsal aorta, remains, forming what is known as aductus Botalli. A rete mirabile on its course is the remnant of the original gill.
The second and third branchial arches are continued as simple trunks into the dorsal aorta, and the blood from the fourth arch mainly passes to the lungs, but a narrow ductus Botalli still connects this arch with the dorsal aorta.
In the Anura the same number of arches is present in the embryo as in the Urodela, all four branchial arteries supplying branchiæ, but the arrangement of the two posterior trunks is different from that in the Urodela. The third arch becomes at a very early period continued into a pulmonary vessel, a relatively narrow branch connecting it with the second arch. The fourth arch joins the pulmonary branch of the third. At the metamorphosis the hyoid artery loses its connection with the carotid, and the only part of it which persists is the root of the lingual artery. The first branchial artery ceases to join the dorsal aorta, and forms the root of the carotid: the so-called carotid gland placed on its course is the remnant of the gill supplied by it before the metamorphosis.
The second artery forms a root of the dorsal aorta. The third, as in all the Amniota, now supplies the lungs, and also sends off a cutaneous branch. The fourth disappears. The connection of the pulmonary artery with both the third and fourth branchial arches in the embryo appears to me clearly to indicate that this artery was primitively derived from thefourth archas in the Urodela, and that its permanent connectionwith the third arch in the Anura and in all the Amniota is secondary.