Book - A Text-book of Embryology 10

From Embryology


Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.

   Text-book of Embryology 1907: 1 Male and Female Sexual Elements - Fertilization | 2 Ovum Segmentation - Blastodermic Vesicle | 3 Germ-layers - Primitive Streak | 4 Embryo Differentiation - Neural Canal - Somites | 5 Body-wall - Intestinal Canal - Fetal Membranes | 6 Decidual Ovum Embedding - Placenta - Umbilical Cord | 7 External Body Form | 8 Connective Tissues - Lymphatic System | 9 Face and Mouth | 10 Vascular System | 11 Digestive System | 12 Respiratory System | 13 Genito-urinary System | 14 Skin and Appendages | 15 Nervous System | 16 Sense Organs | 17 Muscular System | 18 Skeleton and Limbs

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The vascular system, including the blood, the heart, and the blood-vessels, begins its development very early in embryonic life.

While the heart is formed within the body of the embryo, the blood and the earliest blood-vessels have their origin in an extra-embryonic structure, the yolk-sac. It is noteworthy that all parts of the vascular system proceed from mesodermic tissue, the heart and the vessels originating from clefts within this structure, and being lined, therefore, with endothelial cells.

In corres[)ondence with the varying relations which the embryo sustains toward the fetal ap])endages at different times, its circulatory system is distinguished successively by certain special features. Thus, during the activity of the yolk-sac as an organ of nutrition, the vitelline circulation is present; following and supplanting this is the allantoic circulation, which latter, in turn, gives place to, or, in fact, becomes the placental system of vessels.

The Vitelline Circulation and the Origin of the Blood

The seat of the first formation of the blooil- vessels and of the blood is the wall of the yolk-sac, entirely outside of the body of the embryo. The wall of the yolk-sac, the reader may be reminded, consists of the extra-embryonic splanchnopleure covered with a part of the somatopleure. The mesodermic layer of the sac exhibits — at the end of the first day in the chick — a network made up of cords of cells, the angioblast (Fig. 72). Interspersed throughout this network are groups of cells, the substance-islands, which lie within the meshes of the network in relation with the cords of cells composing it (Fig. 72}. Bolli llie c«Us of the cords and of the sulistauce-isbinds are meBenphvmal t-etls. The superficial cells of the cell-cords become fiatttined iu each case tu constitute a conliniions layer \?)iich eocloaes the renininiiig culls ortli^' cjLiI.and ihey thus fnrm the eiidutheliul wall of the future b1m>d*- - vessel. The cells of the sub -I stance-iiilunds move apart

^f^ '^i and acquire prolongation!) W T or processes which intercom's , 1 mnnicate, while a gektin' fl ous or f«raiHuid intercellular

ftnbslanee is formed, thus ~Bi pi^hicing an embryonal connective tissue in relation with the network of developing vessels. The solid "vessels" thus formed acquire luniina — on the second day of inculution in the chick — h_v the penetration of fluid from the surrounding niesodernt, this fluid cmwding the cells a|iart, toward the vessel walls. The channels of the vessels are at fin*t quite irregular, being at some points entirely blockeil, at others merely

  • «iu, B. oncrouchcil upon, by masses

of splicniidal cells in connn'Clion with (he vessel walls. arr ihe blood-iaUndB, the aggrt^itions l^nvvd (h»> fetal nd blo-Hl-eells (Fig. 72). ' ■UpuJ-bdauib multiply by mitotic divi• l«rt, suci-eesively become detached and .■•.■Ml-.stmuit. This process continues until X\m*< ci'ILs the enrthioblasts, the first corpuscular elements of the fetal blood, are at first colorless, but soon become pale yellow. Their formation goes hand in hand with the formation of new blood-vessels. Their color deepens somewhat, liemoglobin developing within the cytoplasm. Their nuclei are hirge and reticular. The majority of them acquire small dense nuclei and are then called normoblasts. The erythroblasts continue to undergo mitotic division in the bloo<l-stream just as they did in the blood-islands, division being seen in the embryo chick up to the sixth day. In man, multiplication of erythroblasts occurs quite largely in early fetal life, particularly in regions where the circulation is slow, as in the liver, the spleen, the bone-marrow, and the lymph-nodes ; while in later fetal life and after birth it takes place in the red bone-marrow only.

It is especially noteworthy that these early fetal bloodcells are nucleated in contradistinction to the adult nonnucleated red. blood-corpuscles ; and that the nucleated form is present throughout life in all vertebrates but mammals.

Up to the end of the first month the nucleated red cells are the only corpuscular elements found in the blooil. In the second month the non-nucleated red blood -disks, the erythrocytes, make their appearance, and either in the third month or very soon thereafter outnumber the nucleated cells. Diflferences of opinion obtain as to the mode of origin of the erythrocyte, but the prevailing view is that it results from the normoblast by the loss of the nucleus of the latter. The nucleus becomes globular and more dense, assuming in some cases a dumb-bell shape, and is extruded from the cell, after which it is thought to undergo partial disintegration and then absorption by leukocytes. Some observers maintain that the nuclei are dissolved within the cell. Nuclei in the process of extrusion have been observed in cat-embryos. After extrusion of the nucleus the remaining cytoplasm of the cell assumes the biconcave form of the adult red blood-corpuscle or erythrocyte.

The origin of the leukocytes is a somewhat unsettled question. They are found in the blood of chick-embryos at the eighth day and in the rabbit-embryo at the ninth day ; in the human embryo they are seen in the second month. It is probable that they originate in the lymph-nodes, the bonemarrow, the liver, and the spleen during fetal life, but after birth only in the bone-marrow, the lymph-nodes, and the spleen. Their birthplace would be, therefore, lymphoid tissue and their ultimate origin mesodermic. It has been suggested that they may be derived from young ery throblasts ; this is denied by Minot. Beard assigns them an entodermal origin, claiming that they are produced by the entodermal epithelium of the thymus and of the tonsil. From the investigations of Engel and of Florence Sabin it would appear that they are first seen in the blood and the lymph-nodes at the same time.

The blood-platelets have been variously interpreted as small nucleated cells and as fragments of broken-down leuk<K?yt(»s. According to the recent work of Wright they are fragments of the processes of the giant cells (myelocytes) of bone-marrow.

Limiting the first network of vessels on the surface of the yolk-sac is a circular vessel, the sinos termiiialis (Plate VI.). Since the yolk-sac is relatively so large that the body of the embryo appears to rest upon it, and since the surrounding Homatopleure is translucent, a surface view of the ovum at this stage shows a vascular zone encircling the embryonic urea and the later Ixnly of the embryo. This zone is the area aaculosa, or vascular area, the seat of the earliest formation of bhxMl and of blood-vessels of the embryo.

Th(» blood-yeBsels originate, as shown above, from the angioblastio network of m<»senchymal cell-cords of the vascular an»a, the cords of cells, at first solid, gnulually becoming hoUowed out to form the vessels. The vascular network at first forni<»d extends by a pnK*ess of budding over the walls of the yolk-sac an<l thence along the vitelline duct into the ImkIv of tln^ embryo. The budding consists in the extension of vessel sprouts or cell-cords — probably from proliferation of the terminal cells of the vessels last forme<l — the sprouts being solid at their ends, since the excavation of a sj^rout alwavs occurs a little later than its forward extension.

Neighboring sprouts communicate with each other to a greater or less extent. In a human embryo of about eighteen days the extension of the vessels — appearing macroscopically as fine red threads — along the vitelline duct is well shown. Having reached the bcxly of the embryo, the vessels take their course to wanl the primitive heart, which has meanwhile been d(;veloping. From the anterior and posterior and lateral limits of the vascular area — using these terms with reference to the axis of the embryonic body — four pairs of vitelline veins converge toward the vitelline duct and unite to form the two vitelline or omplialomesenteric veins. These veins, after entering the body of the embryo, pass headward along the wall of the intestinal tube and empty into the lower or caudal end of the primitive heart. The trunks, which are to constitute the vitelline arteries, after entering the body with the vitelline duct, pass upward along the dorsal body-wall, within the dorsal mesentery, to become continuous with large arterial trunks that have i)roceeded from the primitive heart.

The large trunks referred to are the visceral-arch vessels, which unite to form the primitive aorta?. The visceral-arch vessels (see Fig. 60) are a series of five pairs of arteries that arise by a common stem, the truncus arteriosus, from the upper end of the primitive heart. They pass along the respective visceral arches toward the dorsal surface of the bo<lv where all the vessels of one side unite into a common trunk, the primitive aorta. The two primitive aorta?, passing caudalward in the dorsal mesentery, give off, as their largest branches, the two omphalomesenteric or vitelline arteries above referred to. the development and the regression of the visceral-arch vessels correspond with the growtli and the decadence respectively of the visceral arches. Not all the vessels are present in a fully-developed condition at any one time, the first pair having begun to atrophy before the fifth pair makes its appearance. The metamorphosis into certain adult vessels of such of them as persist will be considered in a later section.

This system of vessels constitutes the vitelline circulation, the manifest function of which is to convey nutritive material from the yolk-sac to the embryo. While the vitelline circulation i.s of great importance in any ovum provided with abundant nutritive yolk, such as that of the bird, it is of (U)mpanitively slight consequence in man and the other higher mauimals, and it must be regarded as a vestige of the avian or reptilian ancestry of the mammalian ovum, or, at Icanf, an a reminder that the mammalian ovum was originally provided with an abundant yolk. It must be borne in mind, how('Ver, that the mammalian blastodermic vesicle imbibes frniM th(! walls of the uterus a richlv nutritive albuminous Ituid, wich may be taken up later and carried to the embryo by \\\i\ vitelline circulation. This system of yolk-sac \v*^ni'\H diHuppears with the regression and disap])earance of the jolt-HHc - -in the human embryo at about the fifth week. The Inlni-i'inbryonic portion of the right vitelline artery persists, hoWdViT, to bc<!onH! the Enipeiior mesenteric artery of the adult. To riMidrr ilhi comprehension of the later phases of the Viiw'nliM' HVMdMU more simple, their consideration is deferred MMlil ihn dnvi'lopnient of the heart shall have been described.

The Development of the Heart

Thn llt!lirt| when Htudied in the lower-type animals, is Minn In bn niorpliologieally a dilatcil and specialized part of a vunriiliir trunk enil>e<lded in the ventral mesentery. In miiiii thi» Hint Inndunient of the heart appeai-s at a very early

Iii!»ioi| nunu'ly, brlore the splanehnopleure has folded in to linn ihn pill t met. or. in other words, before the end of the mM'tiiid sviM'k. Tlii" Inndanu'iit, in all higher vertebrates, is bilalttntl, huviiiK thi^ lonn of two tubes prcKlucecl by vacuolation of tht) nplitnehnie ineHoderm an<l lying widely separated, iin«« in ntifh half of th(« Htill spread out splanehnopleure (Ki^. 7.1), .1). A tniiiHVi'rHe K(*etion through the future neckregion of a hiu'i'p wv nil»l»it-embryo shows tiie tubes cut auritnn, hiiMv ihrir long ii\(*s ans [mnillel with that of the binlv (Kig. 71). With tiu< li>lding in of the splanehnopleure uiid the union of tli«' (ulgi^s of its folds, the tubes are carried tiiward ea4^ii other, and hubse<|Uently, by the disappearance of the tissue intervening b^etween them, their cavities become one (Fig. 73, B ami C). After the formation of the guttract, therefore, and the simultaneous apj^aranco of the ventral body-wall, the heart-fundament is a single straight mesodennic tube, situated in the pharyngeal region, in close relation with the ventral wall of the bmly, between the latter and the fore-gut. Reference to Fig. 73, C, will show that the heart-tube is separated from the bwly -cavity (or coelom) on each side by a layer of the mesoderm, and that these two layers connect the heart dorsiilly with the gut-tract and ventrally with the iMxly-wall, forming rcs|iectivoly the mesocardltun anterioa and the mesocardiam posteriua. These folds temporarily divide the upper portion of the body-cavity into two lateral parts.

Fio, 7S.-SehPin«il OM se on of abb n b 70 n show dcTelnpmcnt of heart: A. embryonfc

e[> ad oHl: fl. more 

kdvanced bUrc, the h[

an hnnp euro p«rt y f d d

C Ep an hnoplourc folded

In to furra gul-tra«. th

e <ancr Slrahl).

The disappearance of the stratum of mesoderm immediately surrounding the heart-tube and tbo differentiation of the tissue limiting peripherally the space thus formed, results in the production of a second larger tube enclosing the first. The cells of the outer tube become specialized

iiit'i miucle-cells, wliicli arc to contitilutc the fiituro heartmiixcle, while those of the inner cylinder Rutteii and assume the endotheliotd type to become the endocardium. The gniwtli of c«ntmlly projecting proceBaes from the muscular wall and th« oiitpoi-kcting of the endothelial tube to cover these proceHHCB ami line the spaces enclosed by thera foreshadow thv tpongy churjctcr of the inner surface of the adult heart, with it8 colnmne camen and omsciili pectinati. It is signifiCaiit, as lihowing tlic contractility of niidiff'erDiitiated protoplasmic cells, that the heart l)^n9 to pulsate even before the ap]>carance of any muscnlar tissue in its walls.

The upper end of the heart-tube ta(>ers away into the truncus arteriosus (Fifi. 75, 4), a vessel which bifurcates into the first jMiir of visceral-arch vessels, while its lower extremity receives llie vitelline veins above referred to. Excessive growth in length, each end of the tube Iwing more or less fixed in position, necessitates flexion or folding, the form which the heart-tul)e assumes in consequence being that of the letter S placed obliquely {Fig. 70, A). The venous

the dorsal wall of the body, with the arterial portion ventral to it, both being brought at the same time into practically one transverse plane by the headward migration of the venous, and the tailward migration of the arterial, moiety. At this time the heart is relatively so large, and the ventral body-wall covering it so thin, that the organ appears as if situated outside of the embryo's body (Fig. 62, p. 116).

Simultaneously with these alterations in position, the arterial part of the heart is being marked off from the Tenons segment by a transverse constriction, the former becoming the ventricle, the latter the auricle or atrium (Fig. 76, A). The narrow communication between the two is the anricnlar or atrioventricular canal, which soon acquires the primitive atrioventricular valves, or endocardial cushions, these being endoeanlial thickenings on the dorsal and ventral walls of the Ciinal. Growing toward cacli other, the cushions meet and unite, forming the septum intermedium (Fig. 78, B,/), which now occupies the middle of the auricular canal, leaving only its lateral portions jwtulous. The truncus arteriosus becomes delimited from the ventricle bv a circular constriction, the fretum Halleri, the proximal part of the truncus arteriosus dilating somcwliat to constitute the bulbus arteriosus. The truncus arteriosus divides into the visceral-arch vessels, as pointed out in the last section.

The Metamorphosis of the Single into the Double Heart

The heart with but one ventricle an<l one auricle or atrium is found not only during the early periods of development in all air-breathing vertebrates, but is the permanent conditi<m in fishes. In the development of the individual, as in the evolution of the higher vertebrate type, the appearance of the lungs, which replace the branchiaj of fishes as an aerating apparatus, is accompanied by a division of the heart into right and left halves for the pulmonary and the general systemic circulation respectively.

The division of the human atrium begins in iho fourth week with the growth of a per|)endicular ridge from its dorsal and cephalic walls (Fig. 78, B), this being indicjited externally by a groove on the outer surface of the corresponding wall of the auricle. The ridgt', gniwing downward, becomeB the septum primiun or auricular septum and fuses with the upper extremity of tiie septum intermediutii of the auricular caiiiil, thus dividiDi; the atrium into the right aiul left auricles (Fig. 77). The atrioventricnlar caual, it^ iiiiterior aud j>osterior cushions having united into the scpuim intermedium, shares in this division, becoming thereby the right and the left auriculo ventricular orifices. The division of the atrium, however, is not as yet complete ; a hiatus, the foramen ovale, exists ventral to the free ventral border of the unittnl septum

tlonornlrlumcoTreipoailliieH'ith •uricuUr appcndagei r, iruneu* arterlasiu; d, nrlculitr canal; c, piimlUve Tcntricte. B, heart of buniBiU embryo of about tbe flllh week (Hli): <i, left auricle: b, rigbt ■aricle: r. trunctu arteiioaiu; d. IntctOTc; e. rtglit veiitrido : /. led TeDtriplc.

primum and septum intermedium. A ridge grows downward from the roof of the atrium U|H)n the right side of the septum pHmum and parallel with it; this is the septum secundum (atrial crescent) and is very much thicker than the primary septum. Its downward growth continues in such manner that it comes t<t bound the foramen ovale vontrally and below, its extremity uniting with the left extremity of the fold which later becomes the Eustachian valve, and thus forming the futuiv atmulus oralis. The psirt of the primary eeptnm which is thus jiartially eiirronnded by the free margin ' the si.'ptuiu secundum [Mjuches into the left auricle to constitutc a fiort of valve for the prcvculioii of regurgitation. At birtli or shortly ufler, the ventral edge of this valve-like fold unites wiih the ventral margin of the foramen ovale, thus obliterating the latter, the fohl beuoming thereby the relatively thin floor tif the fossa ovalis of the adnlt heart.

The diTiBion of the ventride, which follows that of the auricle and wich is completed by the seventh week, is first indicated by a vertieat groove, the snlcns isterrentricalaris, seen on both thedor!*!il uml the ventnil siiHiiee tl'ig. 77), From the internal surface, corresponding to the position of the sulcus, a median centrally projecting ridge appears aud devehipg into a septum (Figs. 7K, /i, and 79, t*), whieli, however, i.s ineompkle above and in front. The deficiency thus left, the ostdiuu interrentriculaie, is obliterated by the tlowngrowth of the aortic septum (Fig. 79, k), upon the coni])letion ' Occasioiiully the foramen ovnla remainit p«tuli)iiit for iicverul weeka ur months nfter birtli or even thnniKhiHit life. Ab thin pondilion ullowx llie venous bliHxl lu minifle willi the arterial, the miKiicu of the Ixily U bluish or rynnntir, uii<) a chliil thus iiltedeil hiii baby."

Fig, 78.— A, BecllOD of liBarl n SpUriiira ; 0. interaurlcular sopluni t, left auricle: f. auricular Miiitl: Embryo of in mm. (Hts): ci, aeiiluin c.moulh or Binm reunicnB; (/.right aurtclo; right ventricle; A, Interventricular septami t nr hamati ctnliryu of alKiiit Hie flflli week '

of which the original single ventricle is divided into the right and the left ventricles. While the interventricular septum of the completed heart is, for the most part, muscular, that portion of it which is produced by the aortic septum always remains membranous, constituting the pars membranacea septi of the adult heart. If this septum is incomplete, as happens occasionally, there is an abnormal communication between the two ventricles.

The truncus arteriosus, after having become somewhat flattened, is divided by the growth of a vertical septum, or partition (Fig. 79, «), into the aorta and the pulmonary artery. The growth of the partition is initiated by the appearance of two ridges on the opposite walls of the truncus, the ridges growing toward each other and finally uniting to form the aortic septum. Two longitudinal grooves which appear upon the surface of the truncus, following the growth of the ridges and corresponding in position with them, indicate the division of the vessel into the aorta and the pulmonary artery. The septum grows downward to meet and unite with the ventricular sej)tum, as indicated above. Though the three septa referred to develop indejwndently of each other there is such correspondence between them, as to position, that the effect is as if they constituted one continuous structure.

Before the divisicm of the atrium into the auricles, its walls pouch out on each side to form the auricular appendages, one of which belongs to each future auricle (Fig. 77). While it is still a straight tube, the heart receives at its venous extremity the two vitelline veins. Subsequently this particular part of the atrium is distinguished as the sinus venosus or sinus reuniens, this being a short thick trunk into which empty, in addition to the vitelline veins, the ducts of Cuvier and the umbilical veins. The mouth of the sinus venosus is guarded by a valve composinl of two leaflets. The right leaflet or fold is continuous above with a ridge on the roof of the atrium, the septum spurium (Fig. 78, a). In the division of the atrium the sinus venosus falls to the right auricle, while emptying into the left auricle is the single pulmonary vein, which is formed by the union of the four pul

AradI W«« il^ •nd^si kaslk

rrom iIm- fnjiH iif llie superior vena ca\-a to the (root Iff llie infrrinr vciui cava.

The U-ft Irafict of iho valve at the month of the tdaiis vcn'iMia hiH-imnii iHrophie, as does also the septuni spiirium ; tl)'- richt ilrviiW into two partfl, one of which becomes the EaitAchUn Yalr« iit the orifice of the inferior vena cava,

while the other forms the valve of Thebesius, or the coronary valve, at the opening of the coronary sinus (the latter being the persistent lower end of the left duct of Cuvier). The Eustachian valve serves to direct the blood from the inferior cava through the foramen ovale so long as that aperture is present. The single pulmonary vein is in like manner incorporated in the wall of the left auricle, the four pulmonary veins in consequence acquiring separate oj^enings into that cavitv.

The Valves of the Heart

Before the division of the atrium and the ventricle into right and left halves, the atrioventricular canal has the form of a transverse fissure, each lip of which is thickened into a ridge (Fig. 79, ^4). These ridges or endocanlial cushions are the primitive valves. When the atrial partition grows down and the ventricular septum grows up, their free edges meet and unite with the ridges, each ridge being thereby divided, on its atrial surface by the atrial or interauricular septum, and on its ventricular aspect by the ventricular septum, into a right and a left half (Fig. 79, B). Since the ridges, at their points of union with the septa, fuse likewise with each other, the original orifice is bisected into the right and left auricnloventricular apertures, the only valves of which are the ridges or cushions in question.

To trace the further development of the fully formed valves, it will be necessary to consider the changes which now take place in the walls of the heart. It has been seen that the inner surface of the heart accjuires a spongy or trabecular structure at a very early stage by the inward projection of nmscular processes from the outer tube and the pouching out of the inner endothelial tube to cover these. The wall of the ventricle in consequence is relatively very thick and is made up largely of a network of fleshy columns, the spaces of which network are lined with the endocardium (Fig. 80, A). While the outer stratum of the ventricular wall now becomes more compact by the thickening of the trabeculae — and, to some extent, l)y their coalescence — the trabeculae in the vicinity of the atrioventricular valves dill

minisii iii thickness and lose their iiniacnlar cliaracter, Iwing replaced by thin cutiucctive-tissiie cords (Fig. 80, B). That port iif the vetitridilLir ividl which surround,* the iitrioventriciilur oriliee and to wlnt-h the onducardial enshions or primitive valves are attached, likewise becomes deprived of ninscle-cells, the remaining connective ti^Bue assuming the form of thin plates. Thc.^TC plates, with the former endocardial cushions attaehed to their edges, constitnte the

Fig. tU.-OdieniD honing diviBlon ul Into a'lfW and pnlmnniitT Brtery wUh thi lateral leaQcla divldini; rtmpectlvely Inlo

permanent auricnlorentricalar valve-leafleta. The strands of connective tissue mentioned above as remaining after the degeneration of certain of the muacle-traheculffl arc the chords tendineae of the adult heart. Attached at one end to the valve-leaflets, their other extremity is continuous with trabeculae that have remained muscular, the adult musculi papillares.

The Bemilunar valves of the aorta and pulmonary artery appear when the truncus arteriosus divides to form those vessels. The orifice of the truncus arteriosus is provided with a valve having four leaflets (Fig. 81, A). By the division of this vessel into the pulmonary artery and the aorta (Fig. 81, £ and C), the lateral leaflets are bisected, the anterior half of each, with the anterior leaflet, going to the anterior vessel — the pulmonary' artery — while each posterior or dorsal half, with the dorsal leaflet, falls within the orifice of the aorta. The resulting disposition of the segments of the aortic and pulmonary valves is such that, in the aorta, two leaflets are situated anteriorly and one posteriorly, while in the case of the pulmonary artery these conditions are reversed (Fig. 81, C). In the fully developed heart, however, it is found that the aorta has two posterior leaflets and one anterior, and that the pulmonary artery presents one posterior and two anterior segments. In the division of the truncus arteriosus, the anterior half, or the pulmonary artery, falls to the right ventricle, and the posterior trunk, the aorta, to the left ventricle, the two ventricles lying side by side. In order, therefore, that the ventricles may accjuire the relative positions which they hold in the adult there must be such a rotation that the left ventricle comes to lie behind the right. This rotation of the heart from right to left necessarily alters the relation of the pulmonary artery, causing it to lie not directly in front of the aorta, but in front and to the left. If one conceives of a rotation of the two vessels from right to left through an arc of 60 degrees around a vertical axis, the altered relation of the pulmonary and aortic leaflets becomes at once intelligible (Fig. 81, C'and I)),

The Allantoic and the Placental Circulation

The development of the allantois and its accompanying system of blood-vessels is simultaneous with the decline of the yolk-sac and the vitelline circulation. Since the allantois is an e vagi nut ion from the giit-trart (see p. 89), it is a Bplanchiipleuric sac, its walls consisting therefore of an entodermic and a mesodermic layer. Blood-vessels develop within the mesodermic stratum as extensions or branchea of previously existing intra-embryonic tninks. These vessels are the aUantoic arteries and veins. The two allantoic arteries are branches of the primitive aorta and leave the body of the fetus, in compiuiy with the neck of the allantois, at the nmbilicus. Having reached the peripheral jiart of the allantois, they break np into a capillary plexus, the extension of which into the villons processes of the false amnion completes the union of that strncture with the allantois to form the trne chorion (Plate III.),

the two allantoic Teina develop prxri passu with the arteries and con\ey the blood from the chorion to the fetus. Entering the body of the fetus through the still lai^ umbilieiil aperture, they find their way along the intestinal tube tn the septum transveisom — which structure may be regarded as the jiriniitivc iliaphrngm — to the region of the heart, where they <ii>en into the ducts of Cuvier. Each duct of Cuvier (Fig. 84, A) is Ibrmed by the union of the primitiye jugular vein with tbe cardinal vein of its own eidc, the cardinal and the jugular veins returning the blimd respectively from the lower and upper [wirts of the trunk. Tliis system of blood-vessels constitutes the allantoic circulation ; it is of great importance in any ovum that is developed outside of the body of the mother, as in the case of birds, reptiles, and fishes, in which classes the allantois is the organ of nutrition from the time that the yolk-sac ceases lo ]>erform that function until development is complete. In man, however, aa in all other mammals except the monotremes and marsupials, the aliant^iic circulation may be looked upon as, in a measure, rudimentiry, since it serves to convey nutriment from the chorion to the fetus only until the formation of the placenta.

The placental system of blood-yessels, apf>cHring in the third month willi the ilcvclopment of the placenta, includes the principal trunks of the former allantoic system, the allimtoic arteries and veins having Iwcimie the umbilical vessels. The two umbilical arteries convey impure blood from the fetus to the placenta, where it circulates through the capillaries of that organ and receives oxygen and nutriment from the blood of the mother. As before stated, there is no intermingling of the fetal and the maternal blood, the two currents being separated by the very thin walls of the capillaries, through which osmosis occurs. The purified blood returns to the fetus through the umbilical veins and reaches the right auricle through the inferior vena cava, a portion of it having passed through the liver. The two umbilical veins which are present for a time fuse subsequently to form a single vein. The complicated details of the arterial and the venous trunks, and the relation of the latter to the development of the liver and its special system of vessels, may be advantageously considered in separate sections.

The Fetal Arterial System

The truncus arteriosus, the large artery which arises from the, as yet, undivided ventricle of the heart, bifurcates into two trunks, the first pair of visceral-arch vessels (Fig. 82, 4). These first visceral-arch vessels, also sometimes called the first aortic arches, run from the ventral surface of the body along the first visceral arches, toward the dorsum, where they curve downward and pass caudalward, one on each side of the median line, in front of the primitive vertebral column. Very soon there arise from the truncus arteriosus below the point of origin of the first vessels, four^ additional pairs of visceral-arch vessels, which similarly pass dorsad along the corresponding visceral arches, and which unite with the dorsal part of the first pair to form the primitive aorta of each side. Each primitive aorta results, therefore, from the confluence of all the visceral-arch vessels of its own side (Fig. 82). The two aortte afterward become merged into a single trunk. At first the principal branches of the aorta are the vitelline arteries. As these latter vessels become

^ It is sometimes stated that there are six viscenil-arch vessels, the fifth of which disappears, so that the vessel here designated the fifth would rei>resent the sixth-arch vessel of the earlv condition as well as of lower forms.

in«inspicuou8, the allantoic or umbiliciil arteries come into prominence as the chief branches. Iinleod, thp iimhilical arteries may be said to be the coiitiniintion of the aorta, since the largest part of the hlooH-stream is ilivcrt«l into them. The nurta profxr continues in the median line as the caudal aorta, which latter is represented in the adult by the middle sacral artery. A branch from the fiftli arch goes to the lungs. So I'lir till' arterial f-ystem of the fetu.s presents an absolutely symmetrical armngenient (Fig. 82). Changes very soon occur, however, which lead to the asymmetrical condition

Fig.— DlwraTiis illiistmliriE arranm

nil-Ill nf prtnimvi)

lieBit Knd aorUa Tch« Imodlfled rrom Allfd Thoinsonl t vIMIllne Ttln* reii.

rntng blood from

vucular area; 2, ieni>ii« AesmeDt of hvart

Qbe: 3, primitive te

lricle;«, irunciw

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Aiirtie M veHcli to csudul pole ol embrro : vllolllno VUG alar «ra».

found in the adult. These changes are due to the atrophy of sonic trunks and the preponderance of others. From the point where the dorsal extremity of the fourth arch joina the fifth, a branch pa.sses to the rudimentary arm (Fig. 83). The first and second arches, except their ventral and dorsal limb.s. undergo ati-opliy. The ventral limbs of the first and eecond arches persist and become the external carotid artery, while their dorsal extremities, with ttic tliird viscend-arch vessel, become tlic Internal carotid artery. The vcnlnil stem of the third arch coic^titiitcs the common carotid. The right

fonrtli-arch vessel hcroTncs tlic liglit BubclaTian, its stre&ni of Iilowl hc'uv onvcycd in the arm Ity ttio hrancli which Ims takcu its origin from the [xniit of junction of thi; dorsal t'DtU of the fourth and fifth arches. This latter branch is therefore the continuation of the subclavian. The ventral segment of the right fourth arcli would be represented in the

The foorth arch of the left into the thorax, itbe

adult by the innominate artery. B assuraea a lower position ; I comes the arch of the aorta. Sinee therightflftharclibecomes I atrophic beyond the point of origin of the right pulmonary [ artery, the dorsil end of the right fourth-arch vessel — the [ future right subclavian artery — loses its connection with the k primitive aorta, and the latter now a]>)>ears as the continuaF tion of the left fourth arch. The ventral stem of the left L third arch, which becomes the future left common carotid, Mi;d

alwo the left subclavian, ivhicti arises from the posterior or tiomil onil of thi' left fmirth ardi, nn? noM- branches of the arcti of llie nwlu. wn llic tniiicus arteriosus becomes divkleil into the aoria anil the piilmnniiry artery, the left fiftharch vessel and the right pnlnionarv- artery are the only branchou of the trnncus that fall ti> the pulmonary artery, all the other vessels W-'m^ connected wilh the aorta. the left ftflli Tisceral-arcli vessel, therefore, is represontinl in the adult hy the pnlmonaTy artery and the ductus arterioaua. The fchd Kings lieing impervious, only a very sniiill piirt of the lilood of the pahnonury artery is sent to them. Tiie larger ]Kirtioii of the hiiMxl passes fnmi the pnlnionarv artery to the aorla through a communicating trunk, the ductus arterioauB, wliich represents the greater part of the left fifth iireh and whiih lM?eomes imjwrvioiis after birth with the establishmeut of the proper pidmonarv circulation.

These tranafiiimations afford an explanation of the different relations of the recurrent laryngeal nerves of the two sides. At first they are symmetrically arranged. The pnenmogastric nerve, as it crosses the ftinrlh visceral-arch vessel, gives off the recurrent laryngeal nerve, the latter winding around the artery from before backward on its way to the larynx. When the left fourth areh becomes the arch of the aorta and sinks Into the cheat, the nerve is carried with it ; hence after this time, the left nerve is found winding around the arch of the aorta.

Anomsloua airanEements of the branches of the aortic arch, as well .ns of the areh itself, are referable to anomalous development of the original system of visceral-arch vessels. For example, if the right fourth arch, which usually becomes liie right snholavian artery, be suppressed from its origin to the point where the anery for the right upper extremity is given off, the bliKMl must find its way into the latter vessel through the dorsal stem of the fourth arch, and this doi-sal stem will then become the right snbclavian artery. In such case, the right subclavian of the adult will be found to arise from the left extremity of the artih of the aorta and to pass obliquely upward to the right side of the neck behind the tracliea and the esophagus.

The Fetal Venous System

The venotlS Sjrstem of the embryo presents several successive phases, corresponding in part with the various stages in the evolution of the arterial system. The first trunks to appear are the vitelline veins. These vessels have their origin in the vascular area on the wall of the yolk-sac in the manner already described in connection with the vitelline circulation. The two vitelline or omphalomesenteric veins, which result from the convergence of all the venous trunks of the vascular area, follow the vitelline duct into the body of the embryo through the still widely open umbilical aperture and take their course head ward along the intestinal canal to open into the caudal end of the primitive heart-tube (Fig. 82,1, 1). At a later period they open into the sinus venosus of the heart, and still later, when the sinus venosus becomes a part of the general atrial cavity, into the atrium itself. Near their termination these veins communicate with each other by anastomosing trunks that encircle the future duodenal region of the intestinal tube. As the yolk-sac diminishes in size and importance, the vitelline veins decrease in caliber, and the umbilical veins, conveying blood from the allantois and subsequently from the placenta, functionally replace them. The proximal parts of the vitelline veins have an important connection with the circulation of the liver, as will be seen hereafter.

The umbilical veins, which are developed in the mes(Klermic tissue of the allantois, pass from the placenta along the umbilical cord and, entering the fetal body at the umbilicus, run at first along the lateml, and later along the ventral, wall of the abdomen toward the heart. Meanwhile there liave been established a pair of venous trunks, the primitive jugular or anterior cardinal veins (Fig. 84, A), to return the blood from the head and the upper part of the trunk ; and a second pair, the posterior cardinal veins, which bring the blood from the lower part of the trunk, and especially from the primitive kidneys. The primitive jugular vein — which represents the external jugular^ of the adult — passing downward along the dorsal region of the neck, nipets the cardinal vein of it» own aide and unites with it near the heart, the short thick trunk thus formed i>eing the duct of Cnvier. The right and left ducts of Ciivier converge and open tt^ther into the sinuB veuoans (siiiiis reunieus) of the heart, which nlao now receives the vitelline veins and the nnihilical veins. Upon the development of Ihc upper and the lower limhs, the (posterior) cardinal vein appears as if formed by the confluence of the internal and external iliap voin.s, while the primitive jugular below the entrance of the subelaviiin vein is designated, with the duct of Cuvicr, the superior vena cava, since, owing to the preponderance of the jugular over the cardinal vein, the Cuvierian duct appears to be a direct continuation of the jugular. At this time, then, there are two superior venre cava;, the terminal parts of wliieh, however, are not exactly symmetrieid, since the left passes around the dorsal or posterior wall of the atrium, owing to the rotatioQ of the heart from right to left.

^ According to Salza (ol)scr nations on guinea-pig) and MaU (observations on human embryo) the external jugular is a secondary vein and the primitive jugular becomes the adult internal jugular vein.

The lower venous tnmks likewise present a symmetrical arrangement. The bilateral symmetry of this stage of the vonuns system, while permanent in fisliec, becomes modified in man to produce the familiar asymmetrical condition of the adult venous trunks by two factors principally — first, the development of an unpaired vessel which is to constitute a part of the inferior vena cava, and second, the atrophy of certain vessels and parts of vessels with a consequent diversion of the major part of their blood-stream into other channels. Associated with these alterations is the evolution of a special set of bIo< id -vessels, the portal venous sfBtem, for the supply of the deveh)ping liver. The development of the portal system, however, may he deferred for separate consideration (see [tage 177).

Wiien the sinus venosns becomes a part of the atrium — constituting thatjiart uf the wall of the adult auricle whieb is destitute of musculi pectinati — the two ducts of Cuvier, or the superior cavse, as well as the veins from the atKloniinal viscera, open by separate orifices into the atrial cavity. An nnpaiied vessel now develops below the heart in the tissue between the primitive kidneys (Fig. 84, A, 1). This vessel is described as growing downward from the ductus venosusnear the point where the latter vessel is joined by the right hejmtic veins (p. 180). It is also described (Lewis) as being formed by the enlargement of the right subcardinal vein, the subcardinal veins being themselves produced by longitudinal anastomoses between veins on their wav from the mesentery to open into the respective cardinal veins. Tiie vessel in question constitutes the upper or cardiac .segment of the inferior vena cava. The lower extremitv of this trunk anastomoses by two transverse branches with the right and the left cardinal veins (Fig, 84, 7?). The cardinal veins of the two sides are furtlu^r connected bv a transverse trunk at their lower extremities and bv one that passes across the vertebral column just below the heart. In like manner the two su{)erior vena) cavje communicate with each other by a transverse vessel, the tranflverae jugular vein, at the upper jmrt of" the thorax, above the arch of the aorta. With the exception of the unpaired trunk which is destined to constitute the upper port of the inferior veua cava, the arrangement of the veins at this time is absolutely symmetrical. The apparently meaningless asymmetry of the adult venous trunks is easily accounted for if one notes the alterations in the course of the blood-current wliich now occur.

Fig. 84.— Schematic representation of the human venous system, with three successive stages of development (after Hertwig): 1, vena cava inferior; '2, cardinal veins; 3, vena azygos major; 4, vena azypos minor; .">, renal veins; 6, external iliac vein; 7, internal iliac vein; H and 9, common iliac veins; 10, early superior vena* cava?; 11, ducts of f'uvier; IJ, primitive jugular vein; 13, internal jugular ; 14, subclavian vein; l."> and IG, right and Irft innominate veins: 17, vena cava superior; 18, coronary vein; ID, duct of Arantius; 20, hepatic veins.

The bloodl-strcam of the left superior vena cava gradually becomes entirely divertetl into the right cava through the transverse jugular vein, and the part of the left cava below this point, being now functionless, shrivels to an impervious cord (Fig, 84, C). This cord or strand of tissue, the remnant of the left superior cava, is found in postnutjd life, in front of the root of the left lung, embedded in a fohl of the eerous layer of the pericardium, the so-ealletl Testigial fold of Marshall, Since the left superior vena cava receives, near its terminalion in the auricle, the lai^e coronary vein, which returns the greater part of the blond from the heart-wall, this ])roximal extremity of the left cava persists as the coronary sinua of the heart. The transverse communicating trunk — the transverse jugular vein — and the part of the left cava above it now constitute the left imiominate vein, the course of which from left to right is thus exphiincd. The left superior vena cava of the fetus is represented in the adult, therefore, by the sinus coronarius, by the atrophic impervious cord lying in Marshall's vestigial fold, by the vertical part of the left innominate vein and by a part of the left superior intercostal vein.

The lowest connecting branch between the cardinal veins enlarges and conveys to the right enrtiinal vein the blood from the left internal and external iliac veins (Fig, S4j, in consc<|uennp of which the part of the left cardinal vein bctiiw ihu kidney undergoes atrophy and, finally, complete ohliloralitiu. The newly-formed transverse trunk is the left common iliac vein. The part of each cardinal vein above the renal region suffbi's an arrest in growth, in consequence of which the blood is diverted from these veins into the transverse anastomosing branches before mentioned as connecting the respective cardinal veins with the lower end of the unpaired caval trunk (Fig. 84, B and C> 5). As a result^ the lower half of the right cardinal vein, now receiving at its distal end the two common iliac veins, becomes directly continuous with the unpaire<l caval trunk, and with it constitutes the inferior vena cava. The inferior vena cava, therefore, is partly an independently formed structure and is partly the greatly developed lower half of the right cardinal vein. The upper half of the right cardinal vein, conveying now a relatively small part of the blood-stream, becomes the vena azygos major, the termination of which in the superior vena cava is explicable when it is borne in mind that the cardinal and the primitive jugular veins, by their confluence, form the duct of Cuvier.

While no part of the right cjirdinal vein suffers complete effacement, the left one, in a part of its course, entirely disappears. All the blood of the left external and internal iliac veins being transported to the right side of the body through the lowest transverse^ trunk — that is, the newly-formed left common iliac vein — the part of the left cardinal vein Ix'low the kidney retrogrades and disappears. The part of the left cardinal above the renal region lagging behind in growth, the blood from the left kidney is conveyed to the inferior vena cava bv the transverse trunk that (M)nnects the cardinal veins in the renal region ; this transverse trunk becomes, therefore, the left renal vein. Sinee the spermatic veins originally emptied into the cardinal veins, it is found, after these transformations, that the right spermatic opens into the inferior vena cava, while the left spermatic is a tributary of the left renal vein. Some anatcmiists, indeed, regard the left spermatic vein as the representative of the lower part of the left cardinal vein of the fetus.

As the left renal vein develops into the channel for the major part of the blood from the left kidney, the portion of the left cardinal vein above this point remains an inconspicuous vessel, and that part of it intervening between the duct of Ciivicr and the cross branch (Fig. 84, C, 4) situated immediately below the heart undergoes total obliteration. The blood ascending through the persisting part of the left cardinal vein must therefore pass across to the upper part of the right cardinal vein, now the vena azrgos major; and the pervious portion of the left cardinal vein, with the transverse trunk referred to, constitutes the vena asygos minor.

The development of the pericardium is so mtimately related with that of the pleuree ind of the diaphragm that an account of it invohc- a de-icription if the c\olution of tho»e structures. By wa\ of fatilitatuig a cc m prehension of the rather complicated details of the procc ^ the reader is reminded that the tube which constitute-, the primitive heart is formed by the coalescence of the t«o tubes prodiced within the splanchnic mesoderm and that this tube and also, for a time, the heart resulting trom it ire embedded vMthin the ventral mesenterj and further that the part ot the ventral mesentery connecting the heart mth the central body-wall in the mesocardium anterius \tliile the fdd passing from the heart td the jut tnct is the mesocardium posterius (Fig. 8.'i, A, and F jr 73 () Tht '•| ice betw n the he irt and the Ixxly-wall is a part of the body-cavity or cwloin (throat-cavity of Xcllliker, parietal cavity of His). The first indication of the separation of this space from the future abdominal cavity is furnished by the appearance of a transverse ridge of tissue growing from the ventral and lateral aspects of the body-wall. This mass is the septum transversum. It bears an important relation to the course of the vitelline and the umbilical veins. As the veins diverge from the bodv-wall to reach the heart, thev carrv with them, as it were, the parietal layer of the mesoderm in w-hich they are embedded, forming on each side a fold that projects mesial ly and dorsal ly (Fig. 85, B and C), the two folds approaching and finally meeting witli the ventral mesentery in the median plane. The septum transversum thus formed contains in the region nearer the intestine a mass of embryonal connective tissue which is called the liver-ridge or prehepaticus from the fact that the developing liver grows into it. Since the septum transversum, exclusive of the so-called liver-ridge, is the primitive diaphragm, it will be seen that the liver, in the early stages of its growth, is intimately associated with the anlage ^ of the diaphragm. The septum transversum partially divides the body-cavity into a pericardiothoracic and an abdominal part, as shown in Fig. 85, J5 and C Near the dorsal wall of the trunk, on each side of the intestine and its mesentery, the septum is wanting, and thus the two spaces communicate with each other by openings that are known as the thoracic prolongations of the abdominal cavity. At this stage, then, the four great serous sacs of the body, the two pleural, the pericardial, and the abdominal, are indicated, but are still in free communication with each other.

The pericardial cavity is the first one of these to be closed off; subsequently the pleural sacs are delimited from the abdominal space. Just as the transverse septum, which partly forms the floor of the thoracic cavity, holds an important relation to the course of the vitelline and the umbilical veins on their way to the heart, so is a vertical septum

  • Anlage, a German word signifying groundwork, or, in embryology, the

first crude outline of an organ or part, has come into use in English writings upon the subject because there is no exact English equivalent for it entirely distinct from each other. It is evident also, that the mesial wall of each sjjace is constituted by the mesocardium posterius and the dorsal mesentery. The hings first appear as two little sacs, connected by a common pedicle, the future trachea, with the upper end of the esophagus. As they grow downward in front of the esophagus and in contact with it, they push the serous membrane before them carrying it away from the esophagus (Fig. 86, B)y and thus they acquire an investment of serous membrane, which is the visceral layer of the pleura. The layer of serous membrane in contact with the body-wall is the parietal layer of the pleura. The lower extremities of the lungs at length come into relation with the upper surface of the liver, from which organ they are finally se|>arat(Ki by the growth of two folds, the pillars of Uskow, from the dorsolateral n»gion of the body-wall. These folds or ridges projecit forward and unite with the earlier formed septum transvcrsum to complete the diaphragm. So far, however, the diaphragm is merely connective tissue, the muscular condition being acquired later by the ingrowth of muscular substance from the trunk. Occjisionally the dorsal or younger part of the diaphragm fails to unite with the ventral or older fundament on one side of the body, leaving an aperture through which a jx)rtion of the intestine may pass into the thoracic cavity. Such a condition constitutes a congenital diaphragmatic hernia.

The heart and its pericardial sac occupy the greater part of the thoracic cavity, while the lungs arc merely narrow elongated organs lying in the dorsal part of this space as shown in Fig. 86, B. As the lungs increase in diameter, they spread out ventrally and gradually displace the parietal layer of the pericardium (Fig. 86, B) from the lateral wall of the chest, (Towding the pericardium forward and toward the median plane of the body (see Fig. 86, C) until finally the adult relationship of these structures is established.

The Portal Circulation

The circulation of the adult liver is peculiar in that the organ is supplied not only with arterial bloo<l for its nutrition but receives also venous blood laden witli certain products of digestion obtained frtmi the aliiuentarj' tract, the spleen, and the pancreas. This venoua blood enters the liver thrtj tin- portal vein ami is designed tfi supply to the gland the tnat«;riats for the jwrforinance of its fjiecial functiuas.

duodenum by trunks that encircle the bowel, these connecting vessels collectively constituting the anniilar sinns (Fig. 87, B and C). The liver originates from a small diverticulum which is evaginated from the ventral wall of the intestinal canal. Growing forward between the folds of the ventral mesentery, this little tubular sac divides and subdivides so as to produce a gland of the compound tubular type. The developing liver is fn)m the first in close relation with the vitelline veins and their ring-like anastomosing branches, and receives its blood-supply from the latter through vessels that are known as the ven» hepaticsB advehentes (Fig. 87, 10, 10). These afferent vessels break up within the liver into a system of capillaries, from which the blood passes through the efferent vessels, the venae hepaticse revehentes, into the terminal parts of the vitelline veins. Thus a part of the blood of the vitelline veins is diverted to the liver and, after circulating through that organ, is returned to them further on to be conveyed to the heart. As the liver, with its increasing development, requires more and more blood, the entire blood-stream of the vitelline veins passes to it, and the parts of these veins l)ctween the vena? hepaticw advehentes and the vena; hepaticK) revehentes become obliterated (Fig. 87, B and (,■), The vitelline veins, therefore, leave the intestinal canal at the duodenal region and traverse the liver on their way to the heart. In this early dage of the development of the livery then, it receives its nutrition from the yolk-saCy through the vitelline veins.

When the yolk-sack undergoes retrogression, as it does about the fifth we(;k, the liver must draw upon the allantoic and the placental vessels for its nutrition. To do this it must acquire connection with the umbilical veins. The latter vessels j)ass upward from the umbilicus along the ventral wall of the body and empty into the sinus venosus of the heart above the site of the liver (Fig. 87, A, 4, 4). The umbilical veins effect communications beneath the liver with the vente hepaticaj advehentes from the vitelline veins. At about this time the left umbilical vein begins to predominate over the right one, the latter retrograding until, in the umbilical

the cava, whereby its tributaries, the vena) hepatiese revehentes, come to empty into the cava, the downwanl growth of the latter carrying downward likewise the terminations of these veins to their normal position as the hepatic veins emerging from the dorsal surface of the liver. Meanwhile the volume of blood flowing through the umbilical vein has increased to such an extent that the liver is no longer able to transmit it to the inferior vena cava, and consequently a part of this blood passes through the ductus venosus, which extends from the portal fissure, along the dorsal surface of the liver. The blood of the umbilical vein is divided, therefore, into two streams — one that enters the inferior vena cava directly through the ductus venosus and one that traverses the liver on its way to the cava.

The portal vein results from the persistence of a part of the vitelline veins. The vitelline veins, as we have seen, anastomose with each other by two ring-like branches that encircle the duodenum. The right half of the lower ring and the left half of the up|>er one atrophy, so that the blood of the vitelline veins makes its way to the liver through the left half of the lower ring and the right half of the upper one (Fig. 87, D). The left half of the lower ring and the now united portions of the right and left vitelline veins immediately below constitute the superior mesenteric vein, which passes in front of the third part of the duodenum, as in the adult^ and which is later joined by the splenic vein ; while the anastomosing portion of the loop and the right half of the upper loop become the portal vein. So long as the yolksac is present, the vein receives blood both from it and from the walls of the intestine. After the disappearance of the yolk-sac, the intestinal and the visceral veins are the sole tributaries of the portal vein.

The Final Stages of the Fetal Vascular System

The circulation of the fetus at birth and the changes ensuing immediately thereafter may now be easily understood. The fetal blood being sent to the placenta through the hypogastric or umbilical arteries, receives oxygen there and is returned to the body of the fetus through the umbilical vein. The latter vessel takes its course upward along the ventral wall of the abdomen to the under surface of the liver, lying here in the anterior part of the longitudinal fissure. In this position the blood-stream of the umbilical vein is divided into two parts, one of which unites w^th the fetal portal vein to enter the liver, while the other passes through the ductus venosus directly to the inferior vena cava. The blood which enters the liver, after traversing that organ, reaches the inferior vena cava through the hepatic veins. Thus, in the one case directly, in the other case by passing through the liver, all the placental blood reaches the inferior vena cava and passes on to the right auricle of the heart.

From the right auricle the blood passes through the foramen ovale to the left auricle, and thence, through the mitral orifice, to the left ventricle. Being driven from the left ventricle into the aorta, it is conveyed through the branches of the aortic arch to the head and the upper extremities. Finding its way into the veins of these parts, it is returned, through the superior vena cava, to the right auricle, from which cavity it passes, through the tricuspid orifice, into the right ventricle. From the right ventricle it goes into the pulmonary artery. Since the lungs are not as yet pervious, or but very slightly so, the current is deflected almost entirely through the ductus arteriosus to the descending aorta instead of going to the lungs. Some of the blood of the descending aorta is distributed to the various parts of the body below the position of the heart, while some of it is sent through the hypogastric or umbilical arteries to the placenta for aeration! It is evident that no part of the fetal blood, except that in the umbilical vein, is entirely pure, the venous and the arterial blood being always more or less mixed.

With the detachment of the placenta at birth, several marked alterations occur. The circulation through the umbilical vein ceases, that part of this vessel which intervenes between the umbilicus and the portal fissure of the liver becoming, in consequence, an impervious fibrous cord, the round ligament of the liver. The ductus venosus likewise suffers obliteration, becoming the ligamentum venosum Arantii. Since the lungs now assume their proper function of respiration, the communication between the right and the left sides of the heart and also that between the pulmonary artery and the aorta cease. Hence, the respective avenues for these communications, the foramen ovale and the ductus arteriosus, become obsolete. There being no further need for the hypogastric (umbilical) arteries, the circulation through them ceases, and they become mere cords of fibrous tissue, whose ])resence is evidenced by two ridges in the j>eritoneum on the inner surface of the anterior wall of the abdomen. The proximal parts of these arteries persist, however, as the superior vesical arteries.