Talk:Book - Quain's Elements of Anatomy

DEVELOPMENT OF THE BLOOD-VESSELS

The Principal Arteries. The Aorta

The most interesting part of this history is that relating to the development of the aorta and the larger vessels arising from it. The double condition of the main trunk of "the aorta has already been referred to as existing in the chick up to near the end of the second day. About the fortieth hour the inedian fusion or coalescence of the two vessels begins to take place in the dorsal region, by their external union, at first in a very limited space, and very soon afterwards by the formation of a perforation through their united walls. The union of the two vessels which begins in the dorsal region extends itself backwards towards the lumbar vertebrae, and when it reaches the place where the omphalo-mesenteric arteries pass out on each side, these vessels, each of which was originally the continuation merely of one of the aortas, appear now as branches of a single and median aorta. The iliac vessels are the next large vessels formed from the hinder part of the aorta. The first vessels belonging to these trunks are not, however, those of the lower limbs, for these are not yet formed ; but the umbilical or hypogastric arteries, developed at a very early period in connection with the allantois, and subsequently attaining to a large size along with the growth of the placenta. As the limbs are formed, arteries are developed in them, and these are branches of the main aorta ; but they are for a long time comparatively small, w'hile the umbilical arteries speedily attain a large size, so that, even up to the conclusion of foetal life, they appear to form the principal part of the two large vessels into which the aorta divides at its lower extremity. The middle sacral artery may be looked upon as the continuation of the median stem of the aorta, and probably originates from a double vessel in the same manner as the aorta itself.

Fig. 591. — Transverse Section through the Dorsal Region op an Embryo-Chick OP THE Second Day (from Foster and Balfour, after His). M, medullary canal ; Pv, proto -vertebral column ; w, rudiment of Wolffian duct in the intermediate mass; Ch, notocliord ; Ao, one of the two aortas ; A, epiblast ; C, hypoblast ; BC, splanchnopleure ; Pp, pleuroperitoneal space.

Fig. 592.- — Transverse section through the Dorsal region op an Embryo Chick, END op Third Day (from Foster and Balfour).

Am, amnion ; m p, muscle plate ; CV, cardinal vein; Ao, dorsal aorta at the point where its two roots begin to join ; Ch, notochord (the line does not quite reach it) ; Wd, Wolffian duct; Wb, commencement of formation of Wolffian body ; ep, epiblast ; so, somatopleure ; Sp, splanchnopleure ; hy, hypoblast. The section passes through the place where the alimentary canal {hy) communicates with the yolk-sac.

The double state of the main aorta when first formed in the foetus was discovered by Serres, and described bj him in his 4tli Memoir on Transcendental Anatomy (Annal. des Scien. Nat., 18130), but was doubted by Von Baer, as Serres's observations did not show the relation of the primitive trunks of the aorta to the pharyngeal vascular arches. The fact of the original double condition was, however, placed beyond doubt by Allen Thomson (Edin. New Philos. Journal, ISoO) by the method of tranverse sections, then fii-st employed as a means of embryological investigation, and the process of median union was traced in full detail. The relation of this process to the occurrence of a permanent double canal in the aorta as a malformation, as described by Vrolik. Schroder van der Kolk and Cruveilhier. and obseiwed in at least one case by Allen Thomson, has already been referred to in vol. i., p. 350.

According to Serres, the vertebral arteries within the cranium are originally separate, and the basilar artery results from their mesial union or fusion in the same manner as occurs in the aorta, and the median union of the anterior cerebral arteries in the forepart of the Circle of Willis is another example of the same process. It seems probable that the internal cross band observed by John Davy in the interior of the basilar artery (Researches Physiol, and Anatom., lSo9, p. 301) may be a remains of the septum or united walls of the two vertebral arteries.

Aortic OX' Branchial Arches

The two primitive arterial arches which lead into the dorsal aorta from the arterial bulb of the rudimentary heart, at the time of the establishment of the first circulation, are the most anterior of a series of five pairs of vascular arches which are developed in succession round this part of the pharynx ; and which, since their discovery by Eatlike in 1S25 (Oken's Jsis, 1825) have been regarded with much interest, as corresponding with those vessels which are the seat of development of the subdivided blood-vessels of the gills in fishes and amphibia. These vascular arches thus exhibit in the amniota, along with the branchial or pharyngeal clefts and visceral plates, a typical resemblance to the structure of gills, although no full development of these respiratory organs occurs in such animals, but they furnish by their various transformations the basis of formation of the permanent pulmonary and aortic stems and the main vessels to which they give rise.

The form and position of the primitive aortic arches, up to the time of their transformation into permanent vessels, is nearly the same in reptiles, birds and mammals ; and the main differences in the seat and distribution of the large permanent vessels are to be traced to changes in the openness and extent of growth of the several arches. The five pairs Of arches do not all co-exist at the same time, for they are developed in succession from before backwards, and by the third day of incubation, or by the corresponding period of the fourth week in the human embryo, when the posterior arches have been formed, already a part of the anterior arches, beginning with the first one, has become obliterated. Each of the first four branchial arches occupies a place in the substance of the pharyngeal or visceral plates, and in front of one of the pharyngeal clefts. The first or anterior is therefore situated in the inferior maxillary plate, and in front of the tympano-Eustachian, or first pharyngeal cleft ; and the fifth arterial arch is placed behind the fourth pharyngeal cleft, and in the substance of the neck, in which there is no distinct bar or plate in the higher animals, but which is the seat of a developed branchial bar in some aquatic animals.

The vessels forming the arterial arches are given off on each side in succession from two short canals, into which the primitive arterial bulb divides immediately in front of the place where it joins the neck. These may be named the lower (ventral) or anterior aortic roots ; and similarly, when they have passed round the wall of the pharynx, the branchial arches unite in succession into a vessel on each side, thus forming the upper (dorsal) or posterior aortic roots.

On the third and fourth days in the chick, and from the fourth to the sixth week in the human embryo, there are still three complete pairs of arterial arches passing round the pharynx, and connected both before and behind with the anterior and posterior aortic roots previously mentioned. The transformations of these arches were in part traced by Von Baer and various other observers, but the fuller knowledge of their changes is due to the later researches of Eatlike (Mem. of

Fig. 593. — Diagram of the Aortic or Branchial Vascular Arches of the Mammal, with their transformations giving rise to the permanent arterial Vessels (accordiug to Ilatlike, slightly altered).

A, Primitive arterial stem or aortic Lnlb, now divided into A, the ascending part of the aortic arch, and P, the pulmonary ; a, the right ; a', the left aortic root ; A', the descending aorta. On the right side, 1, 2, 3, 4, 5, indicate the five Ijranchial primitive arterial arches ; on the left side, I, II, III, IV, the four branchial clefts, which, for the sake of clearness, have been omitted on the right side. It will be observed, that while the fourth and fifth pairs of arches rise from the part of the aortic bulb or stem, which is at first undivided, the first, second, and third pairs are branches above c, of a secondary stem on each side. The permanent systemic vessels are represented in deep shade, the pulmonary arteries lighter ; the parts of the primitive arches, which have only a temporary existence, are drawn in outline only, c, placed between the permanent common carotid arteries ; ce, the external carotid arteries ; ci, c'l, the right and left internal carotid arteries ; s, the right subclavian rising from the right aortic root beyond the fifth arch ; r, the right vertebral from the same opposite the fourth arch ; v', s', the left vertebral and subclavian arteries rising together from the left or permanent aortic root opposite the fourth arch ; P, the pulmonary arteries rising together from the left' fifth arch ; d, the outer or back part of the left fifth arch, forming the ductus arteriosus ; j)n, 2^n , the right and left pneumogastric nerves, descending in front of the aortic arches, with their recurrent branches represented diagrammatically as passing behind, with a view to illustrate the relations of these nerves respectively to the right subclavian arteiy (4) and the arch of the aorta and ductus arteriosus {d).

Vienna Acad., 1857), and although some 'points are still left in doubt, their history may now be given from these observations, and the supplemental illustration derived from the investigation of the various examples of congenital malformation, the greater number of which are manifestly related to variations in the natural mode of transformation. This will be explained by reference to the diagram in fig. 593.

From these researches it appears that the permanent vessels owe their formation to the persiscence of certain of the foetal arches or parts of them, while other arches or portions of them become obliterated and disappear. Thus it is ascertained that in mammals the main aortic arch, which in the adult passes to the left of the trachea and gullet, is formed by the persistence of the fourth embryonic arterial arch of the left side, which not only remains patent, and becomes connected with the aortic stem of the arterial bulb, but keeps pace by its increased width and the development of its walls with the rate of growth in the other parts of the body, so that it soon surpasses all the rest of the arches in its width of calibre and thickness of its walls. In birds, however, the permanent aortic arch passes to the right of the trachea and gullet, and it is formed by the persistence of the fourth embryonic arch of the right side ; while, in all reptiles, as there are two permanent aortic arches, it is by the persistence of both the right and left foetal arches that the two aortas are produced, the right being that which is most directly connected with the systemic or left ventricle.

The pulmonary arteries of mammals would appear by Ratlike's observations to be developed in connection with only one foetal arterial arch, viz., the fifth of the left side, from the middle part of which they appear as branches, and the whole fifth arch of the right side undergoes rapid atrophy and ultimate obliteration. The first part of the left fifth arch, becoming the common pulmonary artery, is connected with that division of the arterial bulb which is separated as the pulmonary stem ; but the remote part of this arch also remains fully patent, and undergoing equally with the rest of it full development, continues to lead into the left root of the aorta as ductus arteriosus Botalli, which serves to convey the blood from the right ventricle of the foetal heart into the descending aorta, but becomes obliterated at the time of birth.

This duct is therefore in mammals due to a persistent condition of the fifth left branchial arch. But, in birds and reptiles, it appears that the process of transformation is somewhat different, for in them the right and left pulmonary arteries (excepting in those serpents in which there is .only one lung developed) are formed in connection with the respective right and left fifth branchial arches, and there are thus two ductus arteriosi during foetal life, the short one of the right side corresponding to that which is left in mammals, and the longer one of the left side passing round the pharynx into the left aortic root. Both of these arches are obliterated at the time of the exclusion of the bird from the eg^ ; but in some reptiles the ductus arteriosi remain permanently open during life.

The subclavian and vertebral arteries were shown by Ratlike to spring from the posterior aortic roots at a place between the junction of the fourth and fifth arches. In mammals, the vessels on the left side are from the first in direct connection Avith the aortic root at the place which they permanently occupy ; but upon the right side, as the fourth arch and the aortic root are obliterated posteriorly, the passage for blood from the aortic stem into the subclavian trunk is formed by the persistence of the forepart of the fourth right arch as far as the place where it meets the origin of the subclavian and vertebral arteries.

The common carotid trunks, occupying the region which afterwards becomes the neck, but which is at first absent or extremely short, are formed by the anterior divisions of the aortic roots ; while the external carotid artery is due to the persistence of a channel in the continuation of each anterior aortic root, and the internal carotid artery arises from the persistence of the crossing third arch and the upper part of the posterior aortic root.

Thus it falls out that, in man and a certain number of mammals, an innominate artery is formed on the right side by the union of the first part of the fourth right aortic arch leading into the right subclavian, and the right anterior aortic root which forms the common carotid ; while, on the left side, the carotid and subclavian vessels rise separately from the permanent aortic arch in consequence of the distance lying between them in the original foetal condition.

It does not come within the scope of this chapter to describe the further steps of development of these vessels, nor to enter into an explanation of the manner in which abnormal position of the arch of the aorta and its branches, or of the pulmonary arteries, may be supposed to arise. For further information on this subject the reader is referred to the short account of the varieties given in the description of the blood-vessels in the first volume of this work, as well as to the third volume of Henle's Handbuch, and to the special works of Tiedemann and Eichard Quain on the Arteries.

DEVELOPMENT OF THE GREAT VEINS

In the early embryo, before the development of the allantois, a right and a left omphalo-mesenteric vem bring- back the blood from the walls of the nmbilical vesicle, and unite to form a short trunk, the meatus venosus, which is continued into the auricular extremity of the rudimentary heart.

In the first commencement of the placental circulation, or in the fourth week of foetal life, two umbilical veins are seen coming from the placenta, and uniting to form a short trunk, which opens into the common omphalo-mesenteric vein. Very soon the right omphalo-mesenteric vein and right umbilical vein disappear. In connection with the common trunk of the umbilical and omphalo-mesenteric veins, two sets of vessels make their appearance in the yomig liver. Those furthest from the heart, named vena; Iwpatlca advehentcs, become the right and left divisions of the portal vein ; the others are the hepatic veins, vencv kcpaticce. Tcvclwnifs. The portion of vessel intervening between those two sets of veins forms the ductnn rcnoKiix, and the part above the hepatic vein, being subsequently joined by the ascending vena cava, forms the upper extremity of that vein. Into the remaining or left omphalo-mesenteric vein, open the mesenteric and splenic veins. The part above the latter forms the trank of the portal vein ; and the portion of vessel between the union of this with the umbilical vein and the origin of the venaj hepaticaj advehentes is so altered that the portal tiimk opens into the commencement of the right vena advehens.

At the time of the commencement of the placental cu-culation, two short transverse venous trunks, the ductii of Ci/ricr, open, one on each side, into the auricle of the heart. Each is formed by the union of a superior and an inferior vein, named the itrimitive jugular and the cardinal.

The primitive jugular vein receives the blood from the cranial cavity by channels in front of the ear, which are subsequently obliterated : in the greater part of its extent it becomes the external jugiilar vein : and near its lower end it receives small brandies, which, grow to be the external jugular and subclavian veins. The cardinal veins are the primitive vessels which return the blood from the Wolffian bodies, the vertebral column, and the parietes of the trunk. The inferior vena cava is a vessel of later development, whichopens into the trunk of the umbilical and omphalo-mesenteric veins, above the vense heijaticas revehentes. The iliac veins, which unite to form the inferior vena cava, communicate with the cardinal veins. The inferior extremities of the cardinal veins are persistent as the internal iliac veins. Above the iliac veins the cardinal veins are obliterated in a considerable part of their course ; their upper portions then become continuous with two new vessels, the j;osfe?-ipr vertchral veins of Ratlike, which receive the lumbar and intercostal twigi.

Fig. 594. — Diagrams illustrating HE DEVELOPMENT OF THE GrEAT Veins (after KiJlliker).

A, plan of the principal veins of the fretus of about four weeks, or soon after the first formation of the vessels of the hver and the vena cava inferior.

B, veins of the liver at a somewhat earlier period.

C, principal veins of the foetus at the time of the first estabhshment of the placental circulation.

D, veins of the liver at the same period.

dc, the right and left ducts of Cuvier ; ca, the right and left cardinal veins ; j. j,ihe jugular veins ; s, the subclavian veins ; az, the azygos vein ; u, the umbilical or left umbilical vein ; «', in B, the temporary right umbilical vein ; o, the omphalo-meseuteric vein ; o', the right omphalo-mesenteric vein ; m, the mesenteric veins ; p, the jjortal vein ; p', p', the vente advehentes ; I, the ductus venosus ; V, I', the hepatic veins ; c'l, vena cava inferior ; il, the division of the vena cava inferior into common iliac veins ; cr, the external iliac or crural veins ; h, the hypogastric or internal iliac veins, in the line of continuation of the jirimitive cardinal veins.

In C, li, in dotted lines, the transverse branch of communication between the jugular veins which forms the left innominate vein ; ri, the right innominate vein ; ca, the remains of the left cardinal vein by whish the superior intercostal veins fall into the left innominate vein ; above lo, the obliquely crossing vein by which the hemiazygos joins the azygos vein.

As development proceeds, the direction of the ducts of Cuvier is altered by the descent of the heart from the cervical into the thoracic region, and becomes continuous with that of the primitive jugular veins. A communicating branch makes its appearance, directed transversely from the junction of the Jeft subclavian and jugular veins, downwards, and across the middle line to the rig-ht jugular ; and further down in the dorsal region between the posterior verteljral veras a communicating branch passes obliquely across the middle line from right to left. The communicating branch between the primitive jugular veins is converted into the left innominate vein. The portion of vessel between the right subclavian vein and the termination of the communicating branch becomes the right innominate vein. The portion of the primitive jugular vein below the communicating vein, together with the right duct of Cuvier. forms the vena cava superior, while the cardinal vein opening into it is the extremity of the great vena azygos. On the left side, the portion of the primitive jugular vein placed below the communicating branch, and the cardinal and i^osterior vertebral veins, together wdth the cross branch between the two posterior vertebral veins, are converted into the left superior intercostal and left superior and inferior azygos veins. The variability in the adult arrangement of these vessels depends on the various extent to which the originally continuous vessels are developed or atrophied at one point or another. The left duct of Cuvier is obliterated, except at its lower end, which always remains pervious as the coronal sinus. Even the adirlt, traces of the existence of this vessel can always be recognised in the form of a fibrous band, or sometimes even a narrow vein, which descends obliquely on the left auricle ; and in front of the root of the left lung there remains a small fold of the serous membrane of the pericardium, the re.sfii/ial fold of the pericardium, so named by Marshall, to whom is due the first full elucidation of the nature and relations of the left primitive vena cava.

Fig. 595. — A and B. — Diagrammatic Outlines op the Vestige of the Left Superigr Cava and of a Case of its Persistence (sketched after Marshall), h,

A, brachio-cephalic veins with the superior intercostal, azygos, and principal cardiac veins.

B, the same in a case of persistence of the left superior cava, showing its communication with the sinus of the coronary vein. The views are supposed to be from before, the parts of the heart being removed or seen through

1, 1', the internal jugular veins ; 2, 2', subclavian veins ; 3, right innominate ; 3', right or reguhir sujjerior cava ; 4, in A, the left innominate ; in B, the transverse or communicating vein between the right and left superior vemg cavte ; 5, in A, the oj)ening of the superior intercostal vein into the innominate ; 5', vestige of the left superior cava or duct of Cuvier ; 5, 5', in B, the left vena cava superior abnormally persistent, along with a contracted condition of 4, the communicating vein : 6, the sinus of the coronaiy vein ; 6', branches of the coronary veins ; 7, the superior intercostal trunk of the left side, or left cardinal vein ; 8, the principal azygos or right cardinal vein ; 7', 8', some of the upper intercostal veins ; 9, the opening of the inferior vena cava, with the Eustachian valve.

The left duct of Cuvier has been observed persistent as a small vessel in the adult. Less frequently a right and left innominate vein open separately into the right am'icle. an arrangement which is also met witli in birds and in certain mammals, and which results from the vessels of the left side being developed similarly to those of the right, while the cross branch remains small or absent. (Quain on the Arteries, plate 58, figs. 9 and 10.)

Fig. 596. — View op the Foetal Heart AND Great Vessels, from the left SIDE, TO SHOW THE VeSTIGE OF THE Left Superior Cava VEI^f in situ. (This figure is j)lanned after one of Marshall's, and slightly altered according to an original dissection.)

a, right auricle ; b, left auricle and pulmonary veins ; c, the conns arteriosus of the right ventricle ; d, the left ventricle ; c, descending aorta ; + , vestigial fold of the loericardium ; /, arch of the aorta, with a part of the pericardium remaining superiorly ; r/, main pulmonary artery and ductus arteriosus ; [/', left pulmonary artery ; 1,1', right and left internal jugular veins ; 2, 2', subclavian veins ; 3, 3', right innominate and superior vena cava ; 4, left innominate or communicating vein ; 5, 5', remains of the left superior cava and duct of Cuvier, passing at + in the vestigial fold of the pericardium, joining the coronary sinus, 6, below, and receiving above the superior intercostal vein, 7 ; 7', 7', the upper and lower intercostal vein, joining into one.

A case is recorded by Gruber, in which the left vena azygos opened into the coronary sinus, and was met by a small vein descending from the union of the subclavian and jugular. (Reichert and Dubois. RejTnond's Archiv, 18(54, p. 729.) In this case, the jugular veins had been developed in the usual manner, while the left vena azygos continued to pour its blood into the duct of Cuvier.

(Consult Kolliker, Entwickelungsgeschichte, p. 41-1, et seq. ; J. Marshall on the Development of the great Anterior Veins in Man and Mammalia, in Phil. Trans., part i., 1850 ; and Wenzel Gruber, Uber die Sinus Communis und die Valvulaj der Vense Cardiaca3, &c., in Mem. de I'Acad. imper. des Scien. de St. Petersbourg, 1SG4 ; and in Virchow's Archiv, Jan. 1SG6.)

PECULIARITIES OP THE FOETAL ORGANS OP CIRCULATION

It may be useful here to recapitulate shortly the peculiarities of structure existing in the advanced stage of the formation of the foetal organs of circulation with reference to their influence in determining the course of the blood during intra-uterine life, and the changes which occur in them in consequence of the establishment of pulmonary respiration at birth.

The so-called foramen ovale retains the form of a free oval opening in the septum auricularum up to the fourth month, but in the course o"f that month and the next there takes place the growth from below and on the left side of a flat plate or curtain, which advancing upwards fills up the floor of the fossa ovalis, adheres to its left borders as far as its anterior part, and then becoming free and passing beyond the anterior border of the fossa, converts the aperture into an oblique passage or slit over the valvular margin of the fold, so that in the last three and a half months the arrangement is completed, by which blood may readily pass from the right into the left auricle, but not in a contrary direction.

The Eustachian Valve constitutes a crescentic fold of the lining structure of the heart, which is situated to the right of the opening of the inferior vena cava and fossa ovalis, deepens that fossa, and directs the blood entering it from the inferior cava towards the opening of the foramen ovale ; while it throws the opening of the great coronary vein into connection with the right auricle, into which the superior vena cava also opens.

The formation at an early period of foetal life of the transverse vein of the neck uniting the left with the right brachio-cephalic veins, carries the whole of the blood returning from the head and neck, together with that from the main azygos, into the stream entermg the heart by the superior cava.

The ductus arteriosus passes from the main pulmonary artery into the aorta, at the hollow part of the arch, a short distance beyond the place of origin of the left subclavian artery. It is nearly of the same width with the pulmonary stem, while the right and left pulmonary arteries are of comparatively small size, so long as the lungs have not been expanded by air in rfespiration.

Umbilical Vessels

Besides the usual branches of the descending aorta intended to supply the abdominal viscera and the lower limbs, two large vessels, named hypogastric or iimhilical arteries, are prolonged from the common iliacs, and passing out of the abdomen, proceed along the umbilical cord, coiling round the umbilical vein to reach the placenta. The commencement of each of these vessels afterwards forms the tmnk of the corresponding- internal iliac artery, but, from their size, they might be regarded in the fojtus as the continuations of the common iliac arteries into which the aorta divides. From the placenta the blood is returned by the umbilical vem. which, after entering the abdomen, communicates by one branch with the portal vein of the liver, and is continued by another, named ductus rcnosus, into one of the hepatic veins, through which it joins the main stem of the vena cava inferior.

Fig. 597. — Semi-diageammatic view of the Organs of Circulation in the Fcetus FROM BEFORE (from Luschka, modified, and from Nature). |

a, front of the thyroid cartilage ; 6, right side of the thyroid body ; c, trachea ; d, surface of the right lung turned outwards from the heart ; e, diai^hragm below the apex of the heart ; /, right lobe of the liver, dissected to show ramifications of the portal and hepatic veins ; /', the middle part and left lobe of the liver in the same manner, .showing branches of the umbilical veins and ductus venosus ; g, right, fj', left kidney ; ij" , suin-arenal bodies ; h, right, li, left ureter ; i, portion of the small intestine tunied towards the side, to show the veins from it going to the portal vein ; Ic, urinary bladder ; I, is placed below the umbilicus, which is turned towards the left of the fcetus, and points by a line to the urachus ; m, rectum, divided and tied at its upjier part.

A, A. right auricle of the heart opened to show the foramen ovale : a probe, introduced through the large divided right hepatic vein and vena cava inferior, is seen passing through the fossa ovalis into the left auricle : at the lower part of the fossa ovalis is seen the Eustachian valve, to the right and inferioi'ly the auriculo-ventricular orifice ; B, the left auricular appendix ; C, the surface of the right ventricle ; D, placed on the inner surface of the left lung, i)oints to the left ventricle.

1, ascending part of the arch of the aorta ; 1', back part beyond the ductus arteriosus ; 1", abdominal aorta; 2, stem of the pulmonary artery ; 2', the place of division into right and left pulmonary arteries and root of the ductus arteriosus : the left pueumo-gastric nerve is seen descending over the arch of the aorta ; 3, superior vena cava ; 3', right, 3", left innominate vein ; 4, stem of the inferior vena cava, between the junction of the hepatic vein and the right auricle ; 4', continuation of the vena cava inferior below ; 5, umbilical vein within the body of the fojtus ; 5 x , without the bo'ly, in the umbilical cord ; 5', 5', ductus venosus ; between 5 and 5', the direct branches of the umbilical vein to the liver ; 5", 5", hepatic veins, tlu-ough one of which a probe is passed into the fossa ovalis and through the foramen ovale ; 6, vena portD3 ; 6', its left branch joining the umbilical vein; (i", its right branch; 7, placed on the right iliac vein, points to the right common iliac ai-tery; 7', left common iliac artery; 8, right, 8', left umbilical arteries coming from the internal iliac arteries ; 8 x , umbilical arteries without the body, in the umbilical cord ; 9, 9', external iliac arteries ; 10, placed below the right renal vessels ; 11, inferior mesenteric artery, above the root of which .ore seen the two spermatic arteries.

Fig. 597,

Course of the Blood in the Foetus

The right auricle of the foetal heart receives its blood from the two venre cavfe and the coronary vein. The blood brought by the superior cava is simply the venous blood returned from the head and upper half of the body ; whilst the mferior cava, which is considerably larger than the superior, conveys not only the blood from the lower half of the body, but also that which is returned from the placenta through the umbilical vein. This latter stream of blood reaches the vena cava inferior, partly by a direct passage — the ductus venosus, and partly by the hepatic veins, which bring to the vena cava inferior all the blood circulating through the liver, whether derived from the supply of placental blood entering by the umbilical vein, or proceeding from the vena portfe or hepatic artery.

The blood of the superior vena cava, descending in front and to the right of the Eustachian valve, and mixed with a small portion of that from the inferior cava, passes on into the right ventricle, and is thence propelled into the trunk of the pulmonary artery. A small part of it is then distributed through the branches of that vessel to the lungs, and retuins by the pulmonary veins to the left auricle ; but, as these vessels remain comparatively undilated up to the time of birth, by far the larger part passes through the ductus arteriosus into the dorsal aorta, entering that vessel beyond the place of origin of the arteries of the head and upper limbs, and, mixed perhaps with a small quantity of the blood flowing into the aorta from the left ventricle, is distributed in part to the lower half of the body and the viscera, and in part is conveyed along the umbilical arteries to the placenta. From these several organs it is returned by the vena cava inferior, the venaj portsB, and the umbilical vein : and, as already noticed, reaches the right auricle through the trank of the inferior cava.

Of the blood entering the heart by the inferior vena cava, only a small part is mingled with that of the superior cava, so as to pass into the right ventricle ; by far the larger portion, directed by the Eustachian valve through the foramen ovale, flows from the right into the left auricle, and thence, together with the small quantity of blood returned from the lungs by the pulmonary veins, passes into the left ventricle, from whence it is sent into the arch of the aorta, to be distributed almost entu-ely to the head and upper limbs. A small portion of it, may, however, flow on into the descending aorta, and join the fuller stream of blood from the ductus arteriosus. From the upper half of the body the blood is returned by the branches of the superior cava to the right auricle, from which its course into the right ventricle and pulmonary trunk has been already traced.

There is probably a considerable difference in the early and more advanced stages of foetal life, m the distribution of the stream of blood entering the heart by the vena cava inferior. In the early stages, a lai-ge part of the cuiTcnt being directed into the left, but in the three last months, and as the foetus approaches maturity, more and more of the blood of the inferior cava joins the stream from the superior cava ; and, indeed, the coui-se of the blood, and the relative position of the veins, as well as other original peculiarities of the foetal heart, become gradually altered, in preparation, as it were, for the important changes which take place at birth. It seems also probable that very little of the blood propelled from the left ventricle passes into the descending aoi-ta beyond the ductus arteriosus diu-ing those months of foetal life in which the peculiarities of the circulation are most complete.

From the preceding account of the course of the blood in the foetus, it will be seen that, whilst the modified blood from the placenta is principally conveyed to the upper or cephalic half of the foetus, the lower half of the body is chiefly supplied with the blood which has already circulated through the head and upper limbs. The larger portion of the blood, however, which passes into the descending aorta, is sent out of the body to the placenta. This duty is principally performed by the right ventricle, which after birth is charged with an office somewhat analogous, in having to propel the blood through the lungs. But the passage of the blood through the vessels of the umbilical cord and placenta is longer and subject to greater resistance than that of the pulmonary chculation, and the right ventricle of the foetus, although probably aided by the left in the placental circulation, also takes a large shave in the systemic through the lower half of the body ; and this, ijerhaps, may be the reason wliy the wall of the right equals in thickness that of the left ventricle in the fa^tus.

Sabatier was the first to call attention particularly to the action of the Eustachian valve in separating the currents of blood entering the right auricle by the superior and inferior venaj cavse. (Traite d'Anat., vol. ii., p. 22i.) This separation, as well as that occm-ring between the currents passing through the aortic arch and the ductus arteriosus into the descending aorta, were illustrated experimentally by John Reid. (See art. '• Heart," in Cyclop, of Anat. and Phj'siol., and Edin. Med. and Sui-g. Journal, ISo.j.) A striking confirmation of the extent to which the last mentioned division of the two currents of the foetal blood may take place, without distm'bance of the chculation up to the time of bii-th, is afforded by the examples of malformation in which a complete obliteration has existed in the aortic trunk immediately before the place of the union of the ductus arteriosus with the posterior part of the aortic arch.

CHANGES IN THE CIRCULATION AT BIRTH

The changes which occur in the organs of circulation and respiration at birth, and lead to the establishment of their permanent condition, are more immediately determined by the inflation of the, lungs with air in the first respiration, the consequent rapid dilatation of the pulmonary blood-vessels with a greater quantity of blood, and the interruption to the passage of blood through the placental circulation. These changes are speedily accompanied by shrinking and obliteration of the ductus arteriosus, in the space between the division of the right and left pulmonary arteries and its junction with the aorta, and of the umbilical arteries from the hypogastric trunk to the place of their issue from the body by the umbilical cord ; — by the cessation of the passage of blood through the foramen ovale, and somewhat later by the closure of that foramen, and by the obliteration of the umbilical vein as far as its entrauce into the liver, and of the ductus venosus within that organ.

The process of obliteration of the arteries appears to depend at first mainly on the contraction of the coats, but this is very soon followed by a considerable thickening of their substance, reducing ra^mlly their internal passage to a minute tube, and leading in a short time to final closure, even although the vessel may not present externally any considerable diminution of its diameter. It commences at once, and is perceptible after: a few respirations have occurred. It makes rapid progress upon the first and second days, and by the third or fourth days the passage through the umbilical arteries is usually completely interrupted. The ductus arteriosus is rarely found open after the eighth or tenth day, and by three weeks it has in almost all instances become completely impervious.

The process of closure in the veins is slower, there not being the same thickening or contraction of their coats ; but they remain empty of blood and collapsed, and by the sixth or seventh day, are generally closed.

Although blood ceases at once to pass through the foramen ovale from the moment of birth, or as soon as the left auricle becomes filled with the blood returning from the lungs, and the pressure ^Yithin the two auricles is equalised, yet the actual closure of the foramen is more tardy than any of the other changes now referred to. It is gradually effected by the union of the forepart of the valvular fold forming the floor of the fossa ovalis with the margin of the annulus on the left side ; but the crescentic margin is generally perceptible in the left auricle as a free border beyond the place of union and not unfrequently the union remains incomplete, so that a probe may be passed through the reduced aperture. In many cases a wider aperture remains for more or less of the first year of infancy, and in certain instances there is such a failure of the union of the valve as to allow of the continued passage of venous blood, especially when the circulation is disturbed by over-exertion, from the right to the left auricle, as occurs in the malformation attending the morbus coeruleus.