1897 Human Embryology 24

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Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History
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Chapter XXIV. Transformations Of The Heart And Blood- Vessels

We have already considered, Chapter X., the origin and early history of the heart and blood-vessels, and have now to consider the metamorphoses of the foetal organs of circulation to the time of birth. We shall take up, 1, the heart; 2, the arteries; 3, the veins.

I. Transformation op the Heart

We left the heart, p. 288, as a median longitudinal tube, with double walls, the inner endothelium, and the outer mesothelial or muscular ; the double tube was free except at its ends, which were attacheil to the walls of the pericardiiun ; the anterior end commimicated with the aortic vessels, the posterior (caudal) end with the veins united in the septum transversum. To develop the adult heart out of this simple tube, five principal sets of changes occur: 1, the bending of the tubular heart; 2, the outgrowth of the auricles; 3, changes in the thickness and histological constitution of the walls ; f, the development of valves; 5, the appearance of secondary partitions dividing the right heart from the left.

The literature upon the heart is xeiy extensive, but the history given by His, **Anat. menschl. Embryonen," Heft III., 129-184, of the human heart, and that given by Bom of the rabbit's heart, 89. 1, are so thorough that I have relied chietly on these two authors. Special mention ought also to be made of Carl Rose's dissertation, 88.1. Besides the special pai)ers'on the heart, there are numerous observations scattered in general works. The general development of the chick's heart is described in the text-books of Kolliker and of Foster and Balfour, and in J. Masius' excellent article, 89.2, based upon models constructed by Bora's method. Of earlier pai)ers that of Lindes, 65. 1, is specially noteworthy, and, as pointed out by His and Born, far in advance of his time.

Bending of the Heart and Formation of the Auricles. — After the disappearance of the mesocardium on both the dorsal and ventral side of the primitive heart, the heart is attached only by its aortic and venous ends. The early enlargement of the j^ericardial cavity has l>een already described ; its size is imix>ii;ant as affording the heart room to elongate, l)end, and enlarge. The straight median heart grows rapidly, and to find rcK>ni bends to the right; in the chick the bending l)ogins at the close of the first day and increases very rapidly duriiig the second day. Fig. '^S:], and tecoming at the same time more complicated by the assumption of an irregular S-shape. In mammals the same form is assumed, and is found in the rabbit at nine days, in human embryos of "^MT) nun., Fig. 284.

The venoiia end of the heart Fig. 284, V.l, lies somewhat to the left and extends for a short distance toward the bead and then passes into the veiitriotilar portion of the tube, which curves, as sliown in the out, 1', /, oii to the ventnil side, where it crosses obliquely to the right side, and then Ix'iiding dorsahranl finally runs towanl the head and becoming naiTOwer patwes into tho bulbils aortee, A.h, or division of the lioart tu)N?, wliich jmsses in the median lino into the'trinik of me aorta. At tliis sbige there \f a very short venous or auricular division -' of the heart, a very long, thick, and much

\ient ventricular division, and n bull»ua di V vision of intennediate dimensions. The dif «■ _^'^ fcrentiation of these divisions comes out more

I t f ^^If'^rly from the study of tho endothelial

Thirij "'^ii^iT"!-.'^.^ 'froM hcHrt (or heart cavities) at this stage, Fig. ciwi/'r"3"iin» ml '"'""" -f^o. The general course of the heart may aimiiu rnniiai «.■■. b- lit be best uuderstootl bv combining tliis Bguro S:r " *■ '"""" ^"" ^" with the prececiing, l-emembering that Fig. i!es a somewhat L-omplex curve

from loft to right, and is then continued lieadward on the right side of the embryo by a very narrow divieion, the fretum Halleri, Fr, which leads into the somewhat wider and curving bulbus aortce. In a slightly older stage. Fig. 38C, the lateral ontgrowths of the auricular division have appeared, T'A, and are the aulages of the true auricular cavities; the two limbs of the ventricle are now nearer together and where they join have a distinct apex, which, owing to the increased bending of the heart, hes a little below (in the figure) the level of the auriclea, Vh. The irregularly S-like course of the heart is very evident in this figure ; one loop of the S is constituted by the auricular division, Vh, the auricular canal, Co, and part of the ventricle y-, the second loop of the S is constituted by the whole of the ventricle, by the fretum Halleri, Fr, and the l>ulbu8, A.h. As the auricular division comprises only alx>ut a sixth of the length of the heart tube and consequently only about a third of one loop of the S, we cannot say that the heart consists of a venous and an arterial loop. This mode of description has unfortunately been often used, and has letl tu much unnecessary confusion.

The heart of the rabbit agrees closely with that of man. Fig. 287 is a side view of the rabbit's heart at nine to nine and one-half days, with the first aortic arch, A\ fidly developed, anil the second, .4', just forming. The model shows esjxicially well the union of the tliree venous tninks of each side in the large median sinus reuniens, which opens into the auricle; lat«r the sinus merges with the right auricle and so disappears as a separate division. It will be noticed ujwn comparison of Figs. 288 and '387 that the muscular heart shows the division of the cardiac tube far less distinctly than does the endothelial heart, nevertheless the auricles, Au, auricular canal, C, and ventricle, V, are perfectly distinguishable in the model of the muscular heart, Fig. 2HT. The most conspicuous of the changes which now follow are, tii-st, the descent of the ventricle and second the enlargement of the two diverticula of the auricles. Both changes are well illustrated by the heart of a rabbit at twelve and one-lialf days^aee Bom, I. c. Fig, 10. Comparison of this figure with the preceding renders evident that the ventricle has descended so as to lie below, (.e, farther tailward, nevertheless the arterial exit of the heart tube (or the transit to the aorta) lies, as before, above or head* ward of the auricles, so that the descent of the ventricle has depended upon or been accompanied by — we may express it either way — tlw lengthening of the bulbus aortae, B.u. Fig. liSS represents the endothelial heart of a human embryo at about the stage we are now considering, and illustrates how the auricles. Ho, enlarge on each side and embrace the bulbus aortse between them, and also how between the two sides of the ventricle the hejirt tube is somewhat constricted, forming, as it were, a narrow passage. Into the space left by this

r.m, Vrnackva nperlcir: s.Mp, Hnitiun npurliiiuof HIh: Ni. araliimMippriiu; ll.flii, aurU via of Mt Hide; .,1 ur, Burit!uUr utul : L.V,

constriction grews tissue from the wall of the muscular heart, which tissue nives rise to the septum inferins, that plays the chief part in the ultimtit« »li vision of the riglit from the left vcntrick'.

We turn now to the consideration of the interior of the heart at a slightlj- more ailvjinced stage, in which the muscular ami ondothelial hearts are clt>sely coii_ioincd, owing chiefly to the growth of the muscular heart having obliterated the spjwe Iwtween it and the endothelium. Fig. 2i^!l exhibits a view of the inside of tlic heart of a human embryo of ten millimetres. The two auricles have expanded so as to meet alK>ve, leaving, however, a ])artitiiin, whii.-h is known as the aeptiiiii .111 i>i'ri 11.1, S.n: U>twcen the two sides uf the ventricle is another jjartly di've!opt?d |wutitioii, prmluewl as just de-ncrilxKl and known as the sejrtum inferius; in the auricular lanal there is also a projecting cushion, \vhi<:h in conjunction with its fellow tends to divide the right from the left side of the auricuUir canal. We thus encounter at three points the commencements of the ultimate division of the heart into right and left sides. The opening of the venous sinus reuniens is no longer in the median line, but upon the right side of the heart, or in other words, into the right auricle. The opening itself is boi*dered by two thin folds or rudimentary valves, of which the lateral one, F.JE', is the anlage of both the valvula Eustachii and the valvula Thebesii, while the medial fold ultimately disappears ; as it exists in the embryo it has been named by His the valvula vestibuli shiistra. Above the venous orifice is a small septum, S.sp, which disappears early in foetal life and is therefore known as the septum spurium. The septum spurium may be regarded as the prolongation upward of the united right and left venous valves. The space between the spurious and the superior septa is named by Born the spatium iuterseptale\ it is indicated for a time by a bulge upon the exterior ; it merges with the general cavity of the right auricle, when the spurious septum disappears. Below the valvula sinistra, and between it and the septiun superius, is the spill i vestibuli, Ai, of His; it is identical with the so-called area interjx)sita of earlier stages. The area interposita is compc^sed of connective tissue and contains no muscle fibres; it is wedgeshaped, and as seen in the interior of the heart, Fig. 289, presents a triangular outline. It belongs, strictly speaking, to the septum transversum, and corresponds to part of the area by which the venous end of the heart is permanently attached to the septum transversum or diaphragm. The septum su^^erius or interauricular partition extends on to the area interjx)sita, and there fades out.

The Primitive Ventricle. — The ventricle is at first simply a bent tube; it may therefore be described as consisting of two limbs, which pass into one another at the apex of the ventricle. Figs. 286 and 288. The connection between the two limbs is originally very narrow, but it early widens out so much that the two limte may be said to fuse into one general cavitj\ This may be called the stage of the primitive ventricle, since it is characteristic of the ichthyopsida. While the two limbs are fusing, the junction with the ventricle of the aorta (fretum Halleri, as the narrow part of the aorta is called) moves toward the median line and takes up its permanent position just in front of the auricular cjmal. The change in position of the l)eginning or ventricular end of the aorta allows the aorta. Fig. 288, C,s, to take a nearly straight course between the auricles. The apex of the primitive ventricle is rounded, and it is not until some time after the heart is completely divided that the ventricle assumes the adult pointed shape.

Changes in the Walls of the Heart.— We consider here, 1, the histogenesis of the hccii't; 2, thickness of the walls; 3, the special connective tissue or non-muscular areas.

1. Histogenesis. — For what little is known concerning the development of muscle fibres see p. 478. The heart consists originally of the endothelial tube and the outer muscular tube. The endothelium^ Fig. 21)0, encU)^ retains its primitive character as a thin layer lining the cavity of the heart, but the exact appearance of the cells at successive stages has still to be ol)served; so far as known the endothelium d()i»s not give oft' any cells to fill up the space between it and the muscular heart. As soon a« the inner surface of the ventricle becomea irregular, Fig. 2!H>, Veil, we find the endothelium clowagainst the muscular wall and following it exactly.

The muscular heart or outer heart tube produces the pericardial covering (mesothelium) of the heart, the muscle fibres, and the connective tisiuc; Fig, 290 illustrates the general course of these modifications. The muscular heart tube begins to thicken, and throws off a certain numl>er of cells which assume a mesenchymal character, and stretch across the space between the outer and inner heart,

as in shown in the huJbus aortSD, Bn, of Fig. 2H0. Gradually the numl>er of thvao cells iucrojisw* until the entire space is (x-cupied and th<' luusculjir lieart is a compact wall reaching to the endothelium. During itn growth we see the muscular heart acquire a more and mori' clcjirly »lifFcnmtiat<'d external layer of (^dothelioid cells; as iiiilicntfil in tli(! figure the layer is (wrticularly distinct in the ten anil 11 lialf days' rabbit over the ventricle. The remaining cells Iteconie far tli(' nvmt luirt nmscle tilires, Imt others retain the mesenchymal chiiracter anil give rise tn the cunnwtive tissue, and [terhnps also to the blood-vessels of the heart — when the blood-vessels first appear in the cardiac walls I do not know.

2. Thickness of the Walls. — From what has been said it is evident that the thickness of the walls depends upon the growth of the muscular heart, which takes place so that each division of the bent heart has its characteristic thickness of wall. In the auricles the walls never become very thick, and are always of about the same diameter throughout, excepting where the veins enter and the heart is attached to the septum transversum. In the auricular canal the walls become considerably thicker than in the auricles, and much less thick than in the ventricles, where the walls are most developed, and form many irregular projections into the interior of the heart so that the tissue assumes a spongy appearance. Fig. 290, Fen, which early becomes one of the most marked characteristics of the ventricles. Q. A. Gibson, 91.1, found that during foetal life the walls of both ventricles are approximately equal in thickness, therefore the thickness of the adult left ventricle is acquired after birth. In the bulbus aortae the walls become a little thicker than in the auricles.

3. NoN-MuscuLAR Areas of the Heart. — There are several spots where connective tissue is developed to the complete or partial exclusion of the muscle-fibres of the outer heart. These spots have great importance in the diflferentiation of the heart. They are : the area interposita; the thickened edge of the septum superius; the bolsters of the auricular canal ; and the ridges in the bulbus aortae.

Sinus Venosus. — A venous sinus, more or less distinct from the auricles and formed by the union of the large veins entering the heart, is foimd temporarily in mammalian embryos, and represents the adult condition of reptiles. At first the sinus, for which His uses the name sinus reumenSy is symmetrically placed in the septum transversum at the venous end of the heart. As soon as the heart has become bent and the descent of the ventricles has occurred, the sinus necessarily lies on the dorsal side of the auricular division of the heart and appears partly free from the septum transversum as a short piece. Fig. 287, between the septum and the auricle. The sinus is long in the transverse direction, narrow in the longitudinal and dorso- ventral direction of the embrs'o. But as the lateral outgrowths forming the auricles are developed, the lateral ends of the sinus are bent headward, so that it becomessomewhat horseshoe-shaped — the convexity being toward the apex of the heart. At the same time the sinus grows much less rapidly than the auricles; thus it becomes proportionately smaller in later stages — in a rabbit of fourteen days its length is ecjual to only half the width of the auricles. Into the ends of the sinus open the ducts of Cuvier, Fig. 287, D.C.y and on each side but nearer the median line the omphalomesaraic and umbilical veins (rablnts of eleven days). The two mesaraic and umbilical openings are, however, soon replaced by a single vein, the vena cava inferior, which o]:)ens on the rif/hf side of the sinus, Fig. !287. The cava inferior is present in rabbits of twelve and one-half days, and its development is described later in this chapter. By the time the cava inferior is develoi)ed, the sinus is no longer found opening into the heart in the median line, but upon the right side, Fig. 287 ; this change Born attributes to the manner in which the partial separation of the sinus from the septum transversmn is effected ; the furrow or groove, which produces the sejyaration, cutting in deeper on the left than on the right side, thus forcing the veins from the left side over to the right. The actual opening of the sinus into the right auricle is elongated and oblique, as shown in Fig. 280, and is bordered by two valves, which unite at the upper end of the heai-t and are continued as the septum spuriuni. The history of the valves is given below, p. 532. The sinus as a whole bulges somewhat into the interior of the auricle. The stage to which we have now traced the sinus venosus is found in the human embrj^o of 10 mm.

In the course of its further development the mammalian sinus merges into the right auricle and entirely disap|)ears as a distinct division. The modification is accomplished very gradually, by the expansion of the right auricle batrkward and downward ; it thus embraces the whole of the right horn of the sinus, converting the right horn into a part of the auricular cavity, and the dorsal or posterior wall of the horn into an integral part of the auricular wall, consequently the valves of the venous opening ap|x?ar to spring from the |n.)sterior wall of the heart. The three permanent lH>d3' ^'^^iiis open as before with a common ol)li(|ue mouth ; compart^ Fig. 289. The upper end of this orifice corrt'sponds to the oi)ening of the vena cava superior dextra, the lower end to the opening of the vena c<iva inferior, the middle to the oi)ening of vena cava superior sinistra. The sinus is found almost completely merged in the auricle in rabbit embryos of about twenty days, and its limits can be traced in considerably older stages, and according to His even in the adult lumian heart.

The left honi of the sinus remains outside the auricle and becomes the coronary sinus of the adult.

Division into Right and Left Hearts. — The developmental conditions which result in the complete division of the heart are established by the primitive l)ending of the heart and the outgrowth of the auricles; the former initiates the division of the ventricle, the latter the final sep^iration of the two auricles. the division is supplemented by that of the auricular canal and aortfi. Accordingly we mav consider the division of the heart under four he^ids: division of, 1, the auricles; 2, the auricular canals; ^5, the ventricle; 4, the aorta.

1. Division of the Auricles. — The histories of the process: given by His and Born, differ in several essential ix)ints. I follow the latter authority, /. c, pp. '50S-:)12, as giving the pri^sumably correct account. When the two auricles gi'ow forth, they expand upward, but there remains between them a partition, Fig. 21) 1, to which His applies the name of septfan snperifi.s. Born the name sepfum priniKtn, As the auricles continue to expand, the septum of course incn»ases by the continued meeting of the auricles, and it also incrcfises, without doubt, by its own growth. The septum early acquires a very characteristic appearance by the thickening of its lower (Hlge, Fig. 2U2, just al)ove the auricular canal; the thin part of the partition contains muscle fibres, but the thic:kened edge

  • I follow Born. ft». 1. 83fi. but init int»'llip*»ntly. fori Imvr U^'n unal>W* to umlerstand fully his account of the nicr^iot; of the siiiUK in the auricles.

Fia. SH.—SectlDD In Ihe FronUl Plane tbrough tbe Heart of ft Bsbblt Embryo of tblrteea Days, cau. Cuulla Buricularls; (.», aepluiii Bpurium; u, Beptuiu superiuB; R V,, right TealrIcle;}W<, pericardial cavity.

consists of embryonic connective tissue; the Beptum is, of course, covered with endothelium. Seen from the side, the edge of the septum presents a curved outline, being concave toward the ventricle. The only connection between the auricle is now under the edge of the septum. This communication has been homolo^zed with the foramen ovale of the fcetal heart in later st^^. Bom has shown that this homology, which was maintained by his predecessors, is incorrect, and that the septum grows down to the auricular canal, C, and by uniting with the partition developed inthecanal closes permanently the primary communication {ostiumprimum of Bom).

The true /oramen ovale is developed as a perforation of the upp^ part of the septum superius. This perforation is teraaed by Bom the ostium secundum, I. c, p. 311. It appears in rabbit embryos of 14 mm. (about tift«en days) ; it is small at first and situated close to the wall of the auricle; as to how it is developed, Bom gives no infomiation. It soon enlarges, and in rabbits of 7.3 Tom. or nearly thirteen days is about the same size as the earlier communication {ostium priinum), which; from this stage on, gradually contracts until in rabbits of 10-12 mm. it has closed. In rabbits of ten roiUimetres (thirteen and one-fourth days) a new septum appears above the foramen ovale; it is crescentic in shape, and belongs to the right auricle, since it springs a little to the right of the insertion of the septum superius. This new partition (septum secundum) was first recognized by Bom, and can be followed a little way alongside the septum superius: it is also distinguished by being thicker than the septum superius; its edge forms part {limbus Vieusenii) of the boundary of the foramen ovale. The foramen ovale remains open during fcetal life, and in man is not completely closed until some time after birth.

On the posterior wall of the auricle the septum superius runs on to the area interposita of His, see p. 525, and can for part of its extent he regarded as an upgrowth of that area. Bom, in opposition to His, attributes little special importance to this relation. The closure of the primary communication between the auricles is better described in connection witli the division of the auricular canal.

3. Division of the Auricular Canal. — The auricular canal in human embryos of 8 mm. is found, as it were, invagioated into the ventricle, Fig, 893, c, c'. There appear also two prominences of connective tissue, one on the posterior, one on the anterior wall of the canal. These prominences are the Endothelkissen of F, F. Schmidt, 70.1, the.Endocarrffcissettof Bom, I.e., 320. They increase in height until they meet and unite (rabbit embryos of about thirteen days) so as to divide the passage of the auricular canal -into two channels, cand c'. The prominences are wide; hence, when they meet, the greater part of the canal is closed and the channels are relatively small. Each channel maintains the direct connection between the auricle and ventricle of its own side, and is triangular in section. The triangular section is a necessary consequence of the mode of formation ; each prominence forms a side, and the original wall of the canal makes the third side. While the prominences are joining one another, the edge of the septum superius also unites with them, so that, except for the open foramen ovale, both auricles and the auricular canal are divided be«no.u.<u...«r, x«a.am«, Art.rw.Hia. ^^^ j,^^ ventricles. His has

proposed so designate the septum thus formed by the term septum tntennedinm, but a special new t«rm seems to me superfluous. As shown in Fig, 292, the septum cufasista of a thinner part between the auricles and a thicker part in the auricular canal. His attributes considerable importance to the area interposita, as contributing to unite the septum superius with the prominences of the auricular canal; compare p. 5^5.

The auricular canal soon ceases to be a recognizably distinct part of the heart, and is represented only by the openings between the auricles and ventricles (osiia airio-ventriculares) , and by the atrioventricular valves.

3. Division of the Ventricles, — The two limbs of the ventricle are, it will be remembered, at first entirely distinct. Fig. 28(i, and even after the ventricle has grown considerably and the connection between the two limbs has widened so much that they form essentially one continuous cavity, the original division Wtween the left limb and the right limb is marked by a groove on the external surface. This groove corresponds to a fold of the canljac wall, and hence is represented in the interior of the heart by a projection which grows, as development proceeds, although the external groove is gradually obliterate<l. The growth of the projection establishes the partition between the ventricles, which is Imown as the septum

Fig. ans.-Oliltiiue Section of the He*r Human Embn-o ol H.S mm. (HIa' Embr; (E. aQHophuoiR ; La. lune: nv. ulniiii venoa itire: i:.A« Ivrt nurlcle; R.Au, right a

inferiiis^ Fig. 292, Sp.i. This septum is thick, and consists chiefly of muscle fibres ; it has a partially trabecular structure, and certain of its trabeculsB are ultimately transformed into chordsB of the atrioventricular valves. In a side view the upper edge of the septum is seen to be curving, the septum as a whole being crescent-shaped ; it is situated somewhat to the right side of the median line, Fig. 291. After it is fully developed (rabbit embryos of 10 mm.) the septiun reaches nearly to the auricular canal and if it were prolonged it would join the right-hand side of the partition in the auricular canal ; on the posterior side of the heart the septiuu does actually join the auricular canal, but on the anterior side it fades out toward the aorta. In brief, the broad communication between the two ventricles becomes an interventricular foramen bounded by the partition of the auricular canal and by the edge of the septum iuf erius ; it repeats for the ventricles the role of the foramen ovale for the auricles, p. 529, but were it to close over, as does the foramen ovale, the left ventricle would have no exit, because, as already described (compare Fig. 288) , the aorta is the prolongation of the right limb of the ventricle. In order to furnish the necessary outlet the aorta is divided into two vessels, and one of these (aorta vera) becomes connected through the interventricular foramen exclusively with the left ventricle, thereby rendering the separation of the ventricles complete. Accordingly, to fully understand this separation we must follow the history of the division of the aorta.

4. Division of the Aorta. — The cardiac aorta comprises the fretum Halleri and bulbuS aortse, which at an early stage differ in the width of their cavities. Fig. 286. This diflference is soon lost, and the cavity (endothelial aorta) becomes flattened except in the truncus aortse or upper part of the bulbus, where the cylindrical form is retained. The plane of the flattened cavity changes; it is sagittal where the aorta arises from the couus arteriosus of the right ventricle, and as we ascend along the aorta we find the anterior edge of the cavity moving toward the left until the plane of the fiattened cavity becomes transverse. Meanwhile the muscular wall of the aortic heart has developed, partly into muscle, partly into connective tissue, and this connective tissue develops into a ridge on each side of the flattened cavity. The ridges increase and unite, thus dividing the aorta into two channels, the anterior or left channel becoming that of the pulmonary artery, the posterior or right channel becoming the permanent true aortic cavity. The union of the ridges begins just where the aorta divides to form the aortic arches, and the partition at this point is sagittal, cf. Fig. 293. The formation of the partition progresses downward toward the ventricle, the plane of the partition gradually changing to transverse. The two ridges are found to extend into the ventricle, and participate in the closure of the interventricular foramen, by developing an oblique partition which grows down to the edge of the septum inferius, and thus converts the interventricular foramen into the orifice of the true aorta. The blood in leaving the left ventricle must now pass through the foramen, then across a space which originally belonged to the right ventricle, but which has been shut off by the down-growth of the septum aorticiun. The ventricular extension of the aortic partition is eflfected chiefly by the left or anterior ridge, the right or posterior ridge passing out more on to the lateral wall of the ventricle where it fades out; the left ridge (rabbit embryos of 14 mm.) runs on to the edge of the septum inferius. The division of the aorta and ventricle is completed in rabbit embryos of about sixteen days.

Fig. 293.— Sections at Different Levels through the Cardiac Aorta of a Human Embryo of 11.5 mm. (His' Embrvo Rf?>. The lowest section is on the left, the highest on the right, a, Aorta; P, pulmonary division, x 15 diams. After W. His.

At the upper end of the aorta the partition extends so that the fifth aortic arches are connected only with the pulmonary aorta, while the remaining arches are connected with the true aorta only. Soon after the internal piirtition is formed, the external division commences as two grooves on the outside of the aorta, beginning just between the fourth and fifth aortic arches. The two grooves extend to the ventricle and gradually deepen, until the aorta is completely divided into two vessels (Bom, /.r., 337), which have, as soon as they are separated, both their connections with the heart and their relative positions to one another essentially as in the adult.

The heart is now completely divided.

Valves of the Heart. —The entrances of the pulmonary veins have no valves; the entrances of the body veins have two valves in the embryo, of which the left disappears and the right persists as the Eustachian valve and Thebesian valve ; the right atrioventricular passage has the tricuspid, the left the bicuspid or mitral valve. The entrances of the pulmonary or right, and true or left aorta are each guarded by three semilunar valves. As is well known, all these valves are set so as to favor the fiow of blood toward the arteries and prevent its fiow toward the veins. We shall consider, 1, the venous valves : 2, the atrioventricular valves : 3, the aortic valves.

1. The Venous Valves. — In a human embryo of 10 mm., the opening of the lx)dy veins or sinus venosus into the right auricle is guarded, as shown in Fig. 289, by two valves or thin fiaps of the he^irt walls; at the upper side of the obliijue opening the two valves unite and are continueKi as the septum spurium, S.sj); the left valve lies near the septum superius and merges into the area interposita; the right valve is from the start much larger than the left, and develops into the valvula Eustachii and valvula Thebesii. The venous valves owe their origin to the sinus venosus l>eing pushed into the right auricle and in consequence forming a fold which projects around the venous orifice into the cavity of the heart. The edge of this fold grows considerably and becomes the anlago of the venous valves.

The leff ralve gradually disappears — i)rol)ably completely or nearly so. But His thought it contributed to fonn part of the edge of the foramen ovale. Bern's later ol)serv'ations. I.e., 331, suggest rather that it never unites with the septum sujierius (inter-auricular partition) but simply aborts, and for a time (embryos of the fourth month) can be recognized as a slight ridge on the posterior wall of the auricle.

The right valve, which is always larger than the left, persists in greater part. Early in its development it begins to grow unequally, so that there is a larger upper flap bounding the main venous openings, and a smaller lower flap bounding the mouth of the coronary vein; the two flaps are, of course, continuous with one another though separated by a notch ; the upper flap is the anlage of the Eustachian, the lower of the Thebesian valve. The Eustachian valve does not include the whole upper division of the primitive valve, for the uppermost part aborte, though it can still be traced in the human embryo of four months and even at seven months (Bom, p. 332).

The septum spurium is to be regarded as the prolongation of the united right and left venous valves. As it contains muscular fibres, its probable fimction is, as su^ested by Bom, to draw the two valves together and prevent the back flow of the blood, a function of great importance in the embryonic heart before the atrioventricular valves are developed. In a human embryo of 34 mm. (beginning of the third month), the septiun is so much reduced that it would not be recognized without knowledge of the preceding stages, and at this time we find the tricuspid and mitral valves in action.

2. The Atrioventricular Valves. — Their development has been studied by Bemays, 76.1, whose results have been confirmed by Bom, 89.1, 340. W. His' observations (" Anat. menschl. Embryonen," Heft III., 152-lGO) also are important. The valves proper — in distinction to the muscles and tendons, which belong to the ventricle — are to be regarded as morphologically modifications of the walls of the auricular canal, the canal being to a certain extent invaginated into the ventricles (W. His, /.c. Fig. 105). Theinvaginated portions of the canal become the anlages of the atrioventricular valves, on the left side the mitral, and on the right the tricuspid. When the auricular canal divides into the two atrioventricular channels, each channel or ostium is triangular in section, and as this form is preserved on the right side of the heart, there are three valves developed, one as the prolongation of each of the three walls of the ostium, but on the left side, in consequence of as yet undetermined conditions, there are developed only two, the mitral valves. In each case the lateral valves are developed from a fold of the heart wall, which, as indicated at x in Fig. 292, is formed partly by the wall of the auricular canal, partly by the wall of the ventricle, and partly by connective tissue in the interior of the fold. The medial valves — one on the left side, two on the right — may be described as prolongations of the septum intermedium. Fig. 292. The muscular trabeculae of the ventricle are, almost from the start, connected with the ventricular surfaces of the atrioventricular valves; out of these trabeculae are developed the chords of the valves, known in the adult as the papillary muscles and chordae tendinesB. The trabeoulsB are originally very irregular in their arrangement, but as development progresses those which are connected with the valves become longer and slenderer, and descend in main lines directly from the valves to the ventricular walls, but preserve the network character. A little later (pig and calf embryos of 45-60 mm.) the valvular trabeculae become very slender though still muscular, in the neig^hborhood of the valves, but toward the apex of the heart the fine trabequlaB imite into plmnp bundles, the papillary muscles (Bemays, 76. 1, 495. At this stage each papillary muscle breaks up into some six or eight muscular cords, which are inserted into the valves. In older embryos (in man during the fifth month) the muscular cords change into tendinous chords; the muscular tissue in them disappears and is replaced by mesenchyma, which becomes fibrillar ; hence each papillary muscle is connected by several filamentous tendons with its valve. The slender tendons are the chordae tendineaB of the human anatomy.

3. The Aortic or Semilunar Valves. — Before the bulbua aortas completely divides into the true and the pulmonary aortae, there appear four small protuberances at its ventricular orifice. Each protuberance is a mass of connective tissue covered by endothelium ; two of them are merely the ends of the ridges, described p. 631, by which the aorta is divided. When the division is completed, the ends of the two ridges are also divided, making four protuberances, or in all six — three for each aortic trunk. These protuberances are the anlages of the semilunar valves, and may be seen in a human embryo of seven weeks. They grow until they meet so as to close the aortic entrances, and assume the adult form by becoming concave. Their exact history has still to be worked out; compare Tonge, 70. 1, 387, on the semilunar valves of the embryo chick.

II. The Arterial System

We left the arterial system consisting of the cardiac aorta, the five aortic arches, and four carotids, the dorsal aorta, vitelline or om Ehalo-mesaraic arteries, and allantoic arteries (see p. 274-27C) and ave now to trace the changes which result in the adult system of arteries — changes \<^hich are verj" numerous.

Aortic Arches. — The general scheme of the metamorphosis of the great arteries of the five gill-arches is indicated by the diagrams, Fig. 204. A is the primitive condition: The wide pharynx, PA, is shaded to suggest its rounded form ; the four gill-clefts of the left side, are also indicated, 1, 2, 3, 4. From the heart, i//, runs out the aorta, which soon forks; each fork gives off five branches, I, II, III, IV, V, one in front of each cleft and a fifth behind the fourth cleft. On the dorsal side the five arches unite into a common tnmk, which joins the corresponding trunk from the opposite side to form the median dorsal aorta, Ao. Now, as the clefts develop from in front backward, so the first branchial arch arises first, the second next, and so on, until the series is completed ; shortly after each arch is formed the aortic vessel appears in it.

The disposition in the human embryo corresponds entirely to the diagram, for the relations are all the same, although, owing to the rolling up of the embryo, the primitive toiX)graphy is disturbed : thus in Fig. 300, we at once recognize the four clefts and the five arches.

The homologies of this complicated aortic system with that of the adult mammal are shown in the diagram Fig. 29i, B. The shaded partB are preserved in the adult; the others disappear. The parts lust are the first and second arches; the dorsed connection betwe^i the third and fourth left arches; the upper part of the left fifth arch; there disappear on the right side the upper part of the fourth and the whole of the fifth arch, and also the dorsal connection of Uie arches with the median dorsal aorta, Ao. There remain parts as follows: 1. The heart aorta, which by an internal septum is divided into two aortae (p. 531), one of which maintains a communication with the right ventricle and is continuous headward with the fifth arch of the left side; from the middle of this arch springs a vessel which soon forks to make the two pulmonary arteries, P; during A

Flo. SM.— A,DiajCTam of Pharym «r ui AmnloCi ofOie pharynx, Pfc; Or. (esophngus: 1, U, III, IV, Ihe aorta nt the heart, HI: on the dorsal side the which Jolna Its opposite fellow to (onn the medial

Vertelwat*, 1, ( Ive archefl again '


ai carotid, sprlnifin^ rrom tbeduntal HliU B emptj'liiK iDio the heart. The airoH-s 1 Tarn ot gllJ-archea as prefl^rvn] In matnnu

Oill pouchea (cleftA ■ — ' — II the torkU

jlDgle tnink,

InvaEinatloii of the eomtral side of the lint - - - - :h:om,

of the

>t aortic arch: oi

fcetal life the upper part of this arch, da, persists as the well-tnown ductus arteriosus, so that there is a direct communication between the pulmonary and the body aorta. Soon aft«r birth the lumen of the ductus is obliterated. 2. The left fourth arch, which is very much enlarged, to constitute the permanent aortic arch ; as shown in the diagram, the obliteration of parts is such that the left fourth arch is the only permanent channel of communication between the heart and the dorsal aorta, Ao; hence the aorta of the adult springs from the heart, and gives off to the right a branch, then makes itself a great arch on the left side up to the back, where it is continued down, i.e. tailward. 3. The third arches on both sides, appearing, as the figure clearly shows they must, as portions of the intern^ carotid, In.c; the ventral stem between the third and fourth arches is the common carotid of the adult on each side, while the continuation of that Btem headward becomes part of the eztemal carotid. 4. The right fork of the aorta beoomes the arteria icnomiiiata, a;

rt of th« right fourth arch rotnains as the right subclavian artery, the corresponding left tiubclavian being given off from the correspooding left arch, that is to say, by the great arch of the aorta. Ill© ventral stem between the right third and fourth arches becomes the common carotid of the right sideBut, though the connections and metamorphoses of the aortic arches are sufficiently illustrated by Fig. 204 to elucidate the homologies, yet the actual course of the arches is somewhat different, Fig. 205, the branching taking place as described p. 30(;. The cardiac aorta at first openH under Sie pharynx between the banes of the manditjular and hj'oid arclies, but by the time the five aortic arches are devclo])ed it has moved tailward ; finally when during the second

Fig. SUB t.nU'rlor W oil of thtr FharTu of a Hummn Luitir) of 8 i mi i 1 njftl 1 to *, Fig. at.— Aonic BjTHlMn of His" Embrro

QIU-pauL-hm; ihH wfcidpniiiil i»nK'hrti«rB tea. Bl, 4.a mm. I-V, AurtlcorchM: I't, luaDdl■TMed by thin walla rrom the .■nKnirmmL ; lfi« Wb; M. Ihyrui.l f\aBA: K. n ■- - ^- "

Sll-andm iihi>w thii aortic «n.'h™ drawn In pulmonary wti-rr : /fl, lutiK; ' ttnl linm aiiil arlnlDK tnim the IwBrt aorta. xWdlaou. AfUv W. His. Ao; it, month: (>f. uwiphapia ; tw, body otvltr. >. W JlaTiiH. AtterHlB.

month the bead is I>ent l)ack or raised, compare Chapter XVIII., Figs. 2iS, '-i'-i'i, and the front of the neck elmigates, the heart remains on the level with the thorax, and the position of the aorta is relatively lowered. The five aortic arches are found in human embryos of 3.i;-3.a mm., and all iiersist for a short time, l)ut as soon as the neck bend l>eyins to develop (embryos of 4 mm.) the diwtiipearance of the first aortic arch occurs, Fig.2!U'., to l)o verj' soon followed by the disappearance of the third arch, but the dorsid [Kirt of these arches i>ersists, as already explained, a.4 the intenial rarutiil, while the ventral part persists as the stem of the external carotid, which gives off in the region of the hyoid arch a branclif and in the regitm of the mandibular arcli a second branch. The bran:-ho8 are ileiiit^ated by HJB("Anat. menschl. Embrj-oneo," Heft III., IxT) as the nrteria lingualia and arteria mariflaris communis rcsiiectivoly. The arrangement witli ^^ij^arches open, the first and second closed, is shown in Fig. ^^p^Rl^^ the third arches antl the left fourth persist, we have next to consider the modificatioii of the right fourth arch {Aorta descendens dextra) , which in embryofi of 3 mm. , and even lesB, is smaller in diameter than the corresponding left arch, a difference which His is inclined to attribute to tne oblique insertion of the cardiac aorta rendering the left arch the more direct continuation of the cardiac aorta. Curiously enough the difference is lost temporarily (embryoaof 7-lOmm.), but becomes very marked again in those of 11-12 mm. Fig. 397, so that it now is hardly more than a branch of the aorta, suppl^'ing the carotid vl, and vertebral arteries, v, of the right side. In an embryo of i:j.8 mm. the right fifth arch has disappeared, and with it the piece connecting it with aorta descendens dextra. The disposition of the main stems persists at this stage, with little change except iu their diameters imtil after birth. The cardiac aorta (aorta ascendeiis) divides into, 1, the smaller left arch (arteria anonyma) which is continued as the arten'a subclavia and gives off as a branch the stem leading to the

first, second, and third arches of the right Fig. 8»7.— Aortic syBimi oi aide; this stem is ih.6 right carotis commu- {JiTm*^^"^^!' ' V\>TOto4° nis; and into, 2, the larger left arch, areas arterr; ao. aorta: 'sri. thyroid aortce, which is homologous with the anony- f'.^ irancus™|)ii'imoiMJi"'",K ma and like it gives off the carotia communis 5fier'v?^&u***' ^^■' *"*""■ and subclavia of its side, and is then continued on to the permanent dorsal aorta. The connection of the anonj-ma {right fourth arch) with the dorsal aorta is preserved for some time.

The history of the fifth arches is given in the section on the pulmonary arteries, p. 538.

Development of the Aortic Wall. — The aorta, like all other blood-vessels, consist at first of a simple endothelium, to which are added the muscular and adventitial watts by differentiation of the surrounding mesenchyma, which begins to condense around the aort^ by the end of the second month, and during the second month the separation of the mesenchymal coat into tunica media and tunica adventitia becomes apparent (see His, " Auat. menschl. Embryonen," Heft III., 198, also Morpurgo, 86.1). Erik Muller's paper, 88.1, describes, strictly speaking, not the origin of the muscular tissue of the aorta, but of the primitive mesenchyma from the inner wall of the primitive segments.

Aortic Arches in Branchiate Vertebrates. — In aquatic vertebrates the aortic arches do not remain as large vessels, but they break up into smaller vessels and capillaries, which are distributeil through the branchial filaments, or respiratory outgrowths of the gill-arches. When this modification occurs the ventral end of each aortic arch acts as the afferent stem (branchial artery) and the dorsal end as the efferent stem (branchial vein) of the gill. It is evident that the branchial veins are morphologically distinct from the true veins, and belong not to the venous, but to the arterial, system.

Evolution of the Aortic Arches. — That there were in the earlyvertebrates more gill-arches than are preserved in the amniota, has been stated already. But the exact number is uncertain, and as there must have been one aortic trunk in each gill-arch the number of the aortic arches is uncertain. It seems, however, probable that there were at least nine as indicated by the structure of marsipobranchs (Julin, 87.3) and Chlamydoselachus (Howard Ayers, 89.1). Indeed, von Boas, 87. 1, has adduced weighty evidence to support his belief that even in amniota the number of aortic arches is six, a belief which Zimmerman, 89.1, has supported. To settle this question must be left to research based upon very extended comparative anatomical and embryological observations.

It is improbable, as Ayers, 89. 1, has demonstrated, that the united dorsal ends of the aortic arches, which form two stems, represent the forward continuation of the median aorta, but that rather there was primitively a median dorsal aorta extending over the pharynx to the hypophysis, and that there were lateral anastomoses which have been preserved while the cephalic median aorta has disappeareil. Ayer's hypothesis, which seems to me well justified, is incompatible with the current notion that the dorsal aorta represents two stems fused in the median line — a notion which has been specially advocated by Macalister {Jour. Anat.and Physiol,, XX,, 103, 188G). The special importance of the question at present is its bearing on the comparison of the arterial systems of vertebrates and annelids.

The abortion of the aortic arches is attributed by general consent to the head-bend, and consequent cramping of the branchial region, but the factors which have caused the modification of the five partially preserved arches of mammals have still to be ascertained. Hochstetter, 90. 1, 577, suggests that the development of a new trunk of supply — the internal mammar>' — for the anterior intercostal arteries may have been concerned in the abortion of the right aortic root and the changed position of the left aortic root, but leaves his thought unexplained.

Internal Carotids. — As indicated in the diagram. Fig. 294, the internal carotids are developed out of the first, second, and third aortic arches; the third arch loses its connection on the dorsal side with the fourth arch, but keeps its connection with the second and first ; there is thus a direct blood-channel from the cardiac aorta to the vessel, which runs from the dorsid end of the first arch to the head and brain, Fig. 294, In. c. The internal carotid of the adult comprises the thii*d aortic arch, the dorsal pjtrt of the second arch, the dorsal part of the first arch, and the whole of the true internal carotid of the embryo. His ("Anat. mensch. Embryonen," Heft III., 102) states that in man the dorsal connection between the fourth and fifth arches is lost duriug the fifth week ; and points out that, as the heart and cardiac aorta descend, the position of the third arch becomes more and more oblique, compare Fig. 298.

Pulmonary Aorta and Arteries. — In Sauropsida the fifth aortic arches are proser\'ed on both sides, in reptik^ completely, in birds partially, but in mammals the fifth arch entirely disappears on the right side and partially on the left, as established by the classic investigations of Heinrich Rathke, 57.1. In all amniota the lungs are supplied by arterial branches spnnginE from the middle of the fifth aortic arch, in Sauropsida on both sides, in mammals on the left side only, Fig. 298, P. The right (itth arch disappears in man very early, but the left persists throughout foetal life. Concerning the development of the pulmonary artery proper, i.e. the branch from the arch to the lungs, Fig. 208, P, little is known. His ("Anat. mensch. Embrj-onen," Heft II., 18C) finds the reptilian condition — the right and left fifth arches, each producing a branch to the lungs — in an embryo of i.i mm. and more distinctly developed in embryos of 5-6 mm. but later both pulmonary arteries are found to spring by a common stem from

the left fifth arch. How f,o. ae. -Aortic SyBWrn ot W. me' Embryo Kg. 11

iho f<hutMra fnmoa nKriiit T mm- m'- Handible; Zg, Confcue: 1-V, aortic archn:

ine Cnange comes aOOUl l ^^_ „nei,„i ^rVry. F\ pulmowr artery; Lg. luoftl

do not know, and 1 have O. cesophsmia; T. truncuH pulmonalls; Ao. aorta. X

found no explanation of it.

The arteries have a special relation to the bronchi, as is explained in the section on the lungs in Chapter XXIX. Returning now to Fig. 298, it will be observed that the pulmonary artery, P, divides the fifth aortic arch into a lower part, T, connected with the heart, and an upper part, V, connected with the dorsal aorta. The lower part is the future trunk of the pulmonary aorta, and as the lungs develop the pulmonary artery increases in calibre until it equals the trunk, T, in diameter. The upper part, V, is known as the ductus arteriosus or ductus Botalli {BotallischerGang) and it remains throughout the fcetal period as an open channel, so that blood from the right ventricle flows in part to the lungs, in part into the dorsal aorta. As stated above, the lumen of the ductus arteriosus disappears soon after birth.

Dorsal Aorta and Its Branches. — There are many valuable observations on the fcetal arteries scattered in the works of the older embryologists, in the descriptions of human embryos (Chapter XVIII.) and in articles dealing with the development of special oi^ns, but these observations have never been collated, nor has any attempt been made, so far as I am aware, to study comprehensively the morphology of the dorsal aorta and its branches. This is the more singular as much labor as been expended upon the aortic arches and veins. An exception has been made in the case of the intersegmental and vertebral arteries, see below.

That the dorsal aorta is formed very early by the ingrowth of the omphalo-moparaic arteries and that these arteries are the primitive branches of the aorta has been already explained. The next branches to be formed are the umbilical or allantoic, which very early acquire a large size and appear as the main branches of the aorta, but the dorsal aorta is prolonged to the tail, and in tailed vertebrates persists as a permanent and considerable vessel (arteria caudalis) but in man it remains only as a small vessel, the sacra media. From the umbilical arteries, as soon as the anlages of the legs appear, arise branches, the iliac arteries, one on each side to supply the corresponding limbs. With the progress of development the iliacs become the main branches, and the allantoic vessels are very much reduced, becoming the relatively small hypogastric arteries of the adult. Of the omphalo-mesaraic or vitelline arteries the left aborts very early, while the right persists, and soon develops the arteria mesenterica superior as a small branch, which ultimately becomes the principal continuation of the main stem.

Intersegmental Arteries. — The first branches of the aorta to appear in the embryo are a series of small vessels, which pass upward and outward on each side of the embryo. One of these vessels is to be found between every adjacent pair of myotomes, and hence they have been called the interprotovertebral arteries. In the region of the pharynx where the aorta is double, each aorta gives rise to the intersegmental arteries of its own side. Farther from the head the

vessels arise in pairs from the dorsal aorta. In longitudinal horizontal {i.e. frontal) sections of the primitive segments the intersegmental arteries show very well, compare Fig. 119, Is. The metamorphoses of the vessels imder consideration have lx?en worked out for the region of the lieail and neck bv Froriep, 86.1 (pp. 80, l)f3, 103, 108, ^139), and Fr. Hochstetter, 90.1, 90.3. There are six intersegmental arteries between the seven cervical segments ; of these the sixth gives rise to the arteria subclavia as a branch. There are also two segmental arteries head ward of the cervical ones; these two lie respectively between the first cervical and the last occipital segments, and between the last and the penultimate occipital segments. Of these eight arteries the first very early aborts, the second gives rise to a vessel which Fig. 8»-R«»con8tructionoftheAr. runs forward in the head to the mid tertpfi of the Head anil Ne<rk of a Rab- i • j .\ ' • i.\ * i. i x* i

bit Embryo at the end of the eleventh brain and there joms thc internal carotid,

5^ .r{e;ia'%7rt^br3i!,T"^S:ii;S: ^ig. 290. A Series of anastomes are riacarot is interna: i>, intereeinnentai now developed between the intersegmcu iv^'J^'aific arc'h«»7i*°«h^^ tal arteries of the neck and united and S?S.^H<2h^ttS^'™°*" •^' ^' enlarged anastomosing vessels, Fig. 299,

Ai\ appear as a prolongation through the neck of the vertebral artery. The intersegmental branches rapidly abort, except the sixth in the neck which pe^sist^ as the stem, Fig. 299, «. c/, of the vertebral artery, and as soon as the fore limb buds out (rabbits of eleven days) sends a branch to it, which becomes the subclavian artery. The artery between the sixth and seventh cervical vertebrae is thus seen to acquire a special importance, as it becomes the stem of the sub-clavian and vertebral arteries of the adult. We also learn that the vertebral artery is the earlier developed, and that, therefore, the subclavian is morphologically a branch of the vertebral artery, instead of the vertebral being a branch of the subclavian, as usually described in human anatomy. The small original intersegmental arteries persist on the dorsal side of the vertebral artery in the neck, and supply in the adult the circulation of the vertebral column. The next following intersegmental arteries, i. e. those between the seventh cervical and first thoracic, and between the first four or five thoracic segments, imdergo a similar change, a secondary longitudinal vessel being developed between them also (rabbits of thirteen days) , and as they disappear, this vessel becomes a branch — intercostahs superior of human anatomy — of the common stem of the vertebral and subclavian arteries. Hochstetter states, 90.1, 577, that the internal mammary arises as a branch of the subclavian at about the same time as the superior intercostal.

The subclavian does not long retain its original position, but enlarges and migrates from the dorsal to the ventral side of the sympathetic ganglion chain (Hochstetter, 90.1, 578-580).

The remaining intersegmental arteries of the thorax are said to give rise to the intercostal arteries.

The vertebral arteries unite in the occipital region (human embryo of 10 mm. according to W. His, /.c, 193) to form the arteria bastlariSj Fig. 245, while further forward they remain distinct, resulting in the development of the circulus Willisii.

Umbilical Arteries. — These acquire a large size in the human embryo and owing to the reduction of the caudal artery (saa^a media) appear as the terminal forks of the dorsal aorta. They curve around past the cloaca, run in the walls of the allantois or anlage of the bladder, to the umbilicus, and thence through the umbilical cord to the placenta. They develop each a branch, which runs to the hind limb as soon as it buds forth. Until birth the umbilical arterj^ persists as the main stem, but after birth, having lost its main function, it ceases to develop and becomes the hypogastric artery of the adult. The branch to the leg (the common iliac) continues to enlarge and after birth becomes more and more the chief vessel, so that the root of the umbilical artery is converted into the beginning of the iliac artery and the hj^pogastric into a branch of the iliac. The precise history of these vessels has still to be worke<l out thoroughly.

III. The Venous System

The Primitive Veins. — By this heading I mean the jugular, cardinal, vitelline, and umbilical veins, or main venous stems of the first completed embryonic circulation. The initial arrangement of the four pairs of trunk veins can be studied in a human embryo of 4.2 mm., Fig. 300. From the head, where it extends to the forebrain and has several branches, comes the jugular vein, Jg, descending nearly to the level of the septum transversum. From the tail comes the cardinal vein — the posterior cardinal of comparative

anatomy — to meet the juguW vein. Only part of the canlinal vein IB drawn in the figure; in reality it extends the whole length of the rump and ends in the tail. In a cross section the cardinal vein ia seen to be situated originally in the splanchnopleure of the embryo, just at the level of the nephrotomeu (or intermediate cell masses). This position being kept brings the vein, as soon as the Wolffian tubules are developed, to lie just above the Wolffian body, and lateral of the aorta, compare Figs. 301, 135, and 137. The jugular vein occupies the corresponding situation in the neck, but at the level of the segments, which in the chick shows an open connection with the Bplanchnocoele {S. Dexter, 91.1), crosses from the splanchnopleure between the myotomes and splanchnoccele to the somatopleure and runs forward to the head.

The jugular and cardinal veins unite forming a common trunk. Fig. 300. Z).C— the ductus Cuvieri — which passes in an oblique, transverse direction in the somatoi.). Beam- pleurc to the anterior edge of the __j:j;j«K»UrT-rd-.-ffl:o6.cr«V/?t'^u^ feP*"in transversum, and there cuHeri: jIih. «diceot ■mnioD: i4i.i-. niiuntoic oonds towaru the median Ventral

veiD ; «ir. iawrnal carotid :/. flnic aiirtlo ari-h ; i; ._ . ■_, ,■

Aa. auricle: VtK. ventrlule; if, liver: oni, 'me tO OUipty intO the VeilOUS TltelllDe rplnr Al. *]Uq(uIc dlTertlculuQi ; ond of the hpnrt" hv wnv fif tho Art, allanloiu irtery. ACWr VV. His. Vim Ol III*; IH.ll.1% DJ way OI ine

Sinus veuosiis. From the yolk-sac come up the two vifeUine (omplmlo-mesaraic) veins, one on each side, om, and from the allaiitois stalk pasH up through the somatopleure the two allantoic veins, also one on each side. Fig. 300, J /. r. A cross section through the nimp shows the difference in situation of the cardinal vein. Fig, 301, C, in the splanchnopleure above the Wolffian body, and the umbilical vein, i'f, in the somatopleure. The umbilical vein empties into the ductus Cuvieri; the vitelline vein into the sinus venosus. For good tigures of the relation of the primitive veins to the ( rabbit's heart, see Bom, 89.1, Taf. XX,, Fig. 15. The veins, as they approach the heart, imisjs liy the anlage of the liver, and as this ni^aii develops it enters into intimate relations with the vessels, which undergo numerous modifications. It will be convenient to consider the changes in the hepatic veins collectively, and therefore we take up first those

changes in the primary veins which are not associated with the development of the liver. But to do this we must present the early history of the vena cava inferior.

Vena Cava Inferior. — This is a large unpaired vessel, which is developed somewat later — in rabbits not until the twelfth day — than the four pairs of primary veins. Our present knowledge of its development rests chiefly upon F. Hochstetter's admirable investigations, 87.2, 88.1, 88.3. It arises as a small vessel from the ductus venosus of the liver and running through the hepatic substance is continued on the right side ventrad of the aorta in the tissue between the two primitive kidneys, Fig. 302, A, ci, to a point a little beyond the aortic origin of the superior mesenteric artery. It gradually enlarges and forms two fine branches, which pass around the aorta and anastomose with the cardinal veins, the comunication being established about at the origin of the renal vein, Fig. 302, A, r. By the thirteenth day the anterior portion of the cardinal vein is nearly aborted. The lower part of the right cardinal appears now as the direct continuation of the enlarged vena cava, and in fact is the anlage of the lower part of the adult cava inferior. Fig. 302, C. By the fourteenth day the rerial veins appear as branches of the cava, and the caudal ends of the two cardinals are united, thus converting the lower branches of both these veins into branches of the cava inferior. But in man this fusion of the cardinal veins does not take place, but instead there is developed a cross anastomosis by which the lower ramifications of the left cardinal become branches of the cava, Fig. 302. C in Fig. 302 represents diagrammatically the permanent condition. The true vena cava inferior extends only to the renal veins, r, which are persistent segmental branches of the cardinal veins ; beyond this point the cava is really the persistent right cardinal vein ; a cross anastomosis, iJcs, becomes the left common iliac, while the terminal branches of the cardinals are converted into the external and internal iliacs on each side, and empty their blood into the right cardinal, or lower segment of the adult cava inferior.

Fig. 302.— Three Diagrams to illustrate the Transformation of the Venous System. After O. Hertwig. (Eacplanations in the text.)

Metamorphoses of the Primitive Veins. — By a series of changes beginning very early indeed in the embryo the four pairs of symmetrically placed veins take on an asymmetrical arrangement. The chief factors of the change are, 1, the development of new cross trunks, which become main stems ; 2, the abortion of parts of the primitive veins; 3, migration of the vessels.

The changes which occur, in the venous sinus have been already indicated ; those which occur in the liver are described in a separate section below.

Changes of the Ductus Cuvieri and their Connections, — We have already noticed the relations of the ductus to the horns of the sinus venosus, p. 527, and the role of the ductus in shutting oflp the pleural from the pericardial cavity, p. 482. The transformation of the ductus begins with a change in their position, their course becoming steeper, in consequence of the descent of the heart, and at the same time they project across the opening of the pleural cavity into the pericardial cavity, and by finally closing across this opening the ductus are enabled to unite with the mediastinum, thus bringing the two veins nearer together. Of the veins supplying the ductus the jugulars continue to develop and with the growth of the head to acquire an increasing importance, while the cardinal veins have their circulation impeded owing to the competition of the vena cava inferior; the preponderance of the jugular is further increased by the vein of the fore limb, the subclavian^ Fig. 302, A. 5, emptying into it. The two sides of the sinus venosus early become asymmetrical, and, owing to the migration of the sinus toward the right side of the heart, p. 527, the right ductus (the future vena cava superior dextra) has a shorter and more direct course to the heart than the left ductus, which has to bend around the left auricle toward the right. The left ductus runs along the coronary groove of the heart, and there receives the coronary vein, concerning the development of which we have no definite information. This may be called the Sauropsidan stage, since it is permanent in all reptiles and birds; but it is said to be retained in certain mammals. In man, however, a further stage is reached by the partial abortion of the left ductus {vena cava superior sinistra). The reduction begins with the development of a cross anastomosis. Fig. 302, B, as, between the two jugulars. The anastomosing vessel, which is the future vena anonyma sinistra, runs obliquely from the left to the right jugular, where the conditions for the return of blood to the heart are more favorable ; the cross vessel enlarges and in the same measure the right ductus enlarges also, with the further consequence that the right cava usurps more and more of the blood from the left jugular. This leads to the gradual closure of the left ductus Cuvieri (cava sinistra) except of the end next the heart, which persists as the vein delivering the vena coronaria into the right auricle. We thus learn that the cardiac orifice of the coronary vein is really the mouth of the vena cava superior sinistra. The development of the valve (valvula Thebesii) of this orifice is described p. 532.

The cardinal veins undergo a similar change to the jugulars, see Fig. 302, C, in that a cross vein appears which takes the blood of the left cardinal into the right, so that the stream of both cardinals is poured into the right ductus Cuvieri (cava 8Up. dextra). In the account of the vena cava inferior it has been explaioed how the lower parts of the two cardinals are changed, and only the upper parts left. As the main function of the cardinals appears to be to maintain the circulation of the WolSSan bodies, the cardinals lose their importance as the bodies abort. They persist, however, in part to give rise to the azygos and hemiazygos veins of the adult, as sufficiently indicated by Fig. 302, C, az, hz\ hz.

Veins of the Hand and Foot.— Fr. Hochstetter, 91.1, has shown that iu all amniota there is a vein {Randvene) which runs around the edge of the hand (or foot) but when the digits appear this " randvene" is divided and gradually disappears. The veins are developed as a uetwork of capillaries, connected with the randvene and the venous trunk of the limb. As the digits grow out, the randvene persists on each side of each digit, but is .interrupted at the apex. The randvene thus gives rise to the digital veins and probably also is continued on the ulnar side as the permanent vein of the arm, and correspondingly on the leg.

Hepatic Veins. — The following account is an abstract of His' researches (" Anat. menschl. Embryonen," Heft III., 200-210). The liver grows out into the septum transversum and by its enlargement coraes very soon into con tact with the vitelline and um bilical veius on their way to the sinus venosus. The hepatic cy linders grow into the veins push ing, however, the vascular endo thelium before them, and Ju idmg the veins into numerous channels which constitute a network of fine branches. The four\e3sels are thus broken up into smaller vessels, but for a while thej per sist in part as larger stems le^d ing from the liver to the sinus venosus. The liver is now sup plied with all the blood from the chorion (placenta) and the \olk sac. This stage is found in a human embryoof 4.25mm Iig 'MKi. The united umbilical veins fiq me rw ngtru< of the allantoic-stalk, AfJ pass J^,'^ M^^'ctSnai ^ uptotheliverinthesomatopleure li iw^r >. Knun of each side of the body: the left U/r 5iEK"or«

iimliilinol ji tt* *i 14 ulri^arli (1iw>i/1 intrtllne i3 rijrht ud b U ol vein t tihw

umDiucai.i'.K .s,isairea(i\ aecia j,„„ ^,„ y iKdisms. AfterWHis. edly larger than the right twth

veins brejik up within or near the liver into small vessels. The two vitelline veins, Vi, run in the splanchnopleure or wall of the intestine and unite just before they attain the liver, then separate and pass around the entodermal inteetinal canal to unite again on its dorsal Bide, making a complete venouH ring ; they then aguin separate and pass back again around the intestine, forming a second complete ring before they oreak up into small hepatic vessels. On the right side the umbilical and vitelline trunks remain separate as they leave the liver, and open separately into the sinus venosus, but on the left side the two trunks unite, as shown in the figure, and empty by a common stem into the venous sinus, In the next stage the lower part of the right umbilical has no longer any connection with the upper part ot the same vessel , and, therefore, since it continues to act as a yV Ar ii venouspath, itsstrcam is directed downward. The left umbilical vein, on the contrary, has increased in size, Fig. 3()4, V.us, and is pnjlonged within the liver by a large stem, which joins the left side of the upper venous ring formed by the vitelline veins, ri. The upper ring is connected by a. newly developed liirge trunk, V.ar., the vpua uact^ndetis, or vena Aranti — as to the origin of which we possess as yet no satisFig. SM. - ReuonBtnKtion or tLe vmoiu factory data. Remnants of the

Tnmka and Lirer of HIh- Embryo R, G mm.

F.p. I^>nal vein: r.H. rticM um^illlusl tHii: portions of the Umbilical Rnd VI irt4?™v*"iu^'V;"i'T"rf^'v™l.^: telline veins, which in the pre blllcallK deitra. The T«wp1« left ^lilte ara viouS Stage tOok the blood from the liver to the sinus venosus, still persists. It will bo seen that the essential difference between tliis stage and the preceding is, that whereas previously all the blood passeil the liver through small vessels, now only part of it flows tlirough small vessels, the rest through large trunks directly to the heart.

The third stage is established by developing the single portal vein out of the two vitelline veins. This is accomplished as indicated by the diagram, Fig, 3u4, which is to he coinijared with the previous figure. The left side of the upper ring formetl by the vitelline veins, Fig. 303, vi, and the riglit side of the lower ring ixsrsist, leaving parts of each ring to form a single continuous vessel, the rc/Ktpoc/w, which from its mode of origin necessarily makes one ctmiplete spiral turn around the intestine. Herewith the condition is reached which persists throughout ftetal life. Fig. SO."). The portal vein and left umbilical vein supplj- the liver with venous blood, and also form within the liver near its lower surface two large stems which unite and are continued forward by the single vena Arautii. these three great veins after the thinl month are found to lie near the median plane, and to follow straighter courses than in Fig. 3n,i.

The final stage is not roiiched until after birth, when the umbilical vein rapidly aborts. A little later the large channel formed within the liver by the venie portf© and Arantii also diHaiti>ears, except that the part between the union of the vena cava inferior with the vena Arantii (ductus venoeus) and the heart is retained and functions as the cardiac end of the adult cava inferior. In the fourth or adult stage, the Hver is supphed by the portal vein, the representative of the vitelline or omphalo-mesamic veins of the embryo, and all the portal blood passes through the liver in small vessels (capillaries), though, of course, larger venous branches persist to distribute the blood to, and collect it from, the capillaries of the hepatic lobules.

Pulmonary Veins. — It was first shown by Fr. Schmidt, 70.1, that the pulmonary veins are four vessels, wliich unite into a short common stem emptying into the left auricle. Their history has been further elucidated by His, 87.3, 103, and G. I Bom, 89.1, 313, 334. The comm stem appears first as a capillary sel arising from the left auricle near the interauricular septum (twelve days' rabbits) ; the small vessel runs through the mesocardium posterius directTj- toward the anlage of the lungs; by enlarging and branching this vessel forma the system of the pulmonary veins, but for some time after its appearance it remains small. The development is not the same in the rabbit and in man; in the latter the connnon stem enlarges and merges into the auricular cavity, at of the first as a recess, later without demar- syaiem of nia' Embryo r«, n.s min. cation; hence the four pulmonary veins open into the heart By two orifices, the two veins on each side uniting before they empty. Still later (two months' embryo) the four veins each open separately, more of the vein being annexed by the heart. In the rabbit the primitive condition is permanent, and the four pulmonary veins imito before joining the heart.

The course of the lour veins in the lungs has been described by His, 87.3, 103. They run from the central stem one to each lobe of the lung; in other words, from the start there is an upper and a lower vein in each lung; the pulmonary veins are situated below the forking of the trachea, and this relative position the main stems retain throughout life,^-compare Fig. 459.

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Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History

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