permanent right and left aorta; in Birds the right one remains as

Fig. 477.-Diagram of young embryo and its vessels, showing course of circulation in the umbilical vesicle; and also that of the allantois (near the caudal extremity), which is just commencing. (Dalton.)

the permanent aorta, curving over the right bronchus instead of the left as in Mammals.

In Mammals the left fourth aortic arch develops into the permanent aorta, the right one remaining as the subclavian artery of that side. Thus the subclavian artery on the right side corresponds to the aortic arch on the left, and this homology is further confirmed by the fact that the recurrent laryngeal nerve hooks under the subclavian on the right side, and the aortic arch on the left.

The third aortic arch remains as the internal carotid artery, while the fifth disappears on the right side, but on the left forms the pulmonary artery. The distal end of this arch originally opens into the descending aorta, and this communication (which is permanent throughout life in many reptiles on both

sides of the body) remains throughout foetal life under the name of ductus arteriosus: the branches of the pulmonary artery, to the right and left lung, are very small, and most of the blood which is forced into the pulmonary artery passes through the wide ductus arteriosus into the descending aorta. All these points will become clear on reference to the accompanying diagram (fig. 476).

As the umbilical vesicle dwindles in size, the por

Fig. 478.-Diagram of embryo and its vessels at a later stage, showing the second circulation. The pharynx, cesophagus, and intestinal canal have become further developed, and the mesenteric arteries have enlarged, while the umbilical vesicle and its vascular branches are very much tion of the omphalo-mesen

reduced in size. The large umbilical arteries are seen passing out in the placenta. (Dalton.)

teric arteries outside the

body gradually disappears, the part inside the body remaining as the mesenteric arteries.

Meanwhile with the growth of the allantois two new arteries (umbilical) appear, and rapidly increase in size till they are the largest branches of the aorta: they are given off from the internal iliac arteries, and for a long time are considerably larger than the external iliacs which supply the comparatively small hind-limbs. Veins. The chief veins in the early embryo may be divided

Fig. 479.-Diagrams illustrating the development of veins about the liver. B, d c, ducts of Cuvier, right and left; e a, right and left cardinal veins; o, left omphalo-mesenteric vein; o, right omphalo-mesenteric vein, almost shrivelled up; u u umbilical veins, of which u', the right one, has almost disappeared. Between the venae cardinales is seen the outline of the rudimentary liver with its venæ hepaticae advehentes, and revehentes. D, ductus venosus; ", hepatic veins; ei, vena cava inferior; P, portal vein; P' P', venæ advehentes; m, mesenteric veins. (Kölliker.)

into two groups, visceral and parietal: the former includes the omphalo-mesenteric and umbilical, the latter the jugular and cardinal veins. The former may be first considered.

The earliest veins to appear in the foetus are the omphalomesenteric or vitelline, which return the blood from the yolk-sac to the developing auricle. As soon as the placenta with its umbilical veins is developed, these unite with the omphalo-mesenteric, and thus the blood which reaches the auricle comes partly from the yolk sac and partly from the placenta. The right omphalomesenteric and the right umbilical vein soon disappear, and the united left omphalo-mesenteric and umbilical veins pass through the developing liver on the way to the auricle. Two sets of vessels make their appearance in connection with the liver (venæ hepaticæ advehentes, and revehentes), both opening into the united omphalo-mesenteric and umbilical veins, in such a way that a portion of the venous blood traversing the latter is diverted into the developing liver, and, having passed through its capillaries,

returns to the umbilical vein through the venæ hepatica revehentes at a point nearer the heart (see fig. 479). The portion of vein between the afferent and efferent veins of the liver becomes the ductus venosus. The venæ hepaticæ advehentes become the right and left branches of the portal vein, the venæ hepatica revehentes become the hepatic veins, which open just at the junction of the ductus venosus with another large vein (vena cava inferior), which is now being developed. The mesenteric portion of the omphalo-mesenteric vein returning blood from the developing intestines remains as the mesenteric vein, which, by its union with the splenic vein, forms the portal.

Thus the fœtal liver is supplied with venous blood from two sources, through the umbilical and portal vein respectively. At birth the circulation through the umbilical vein of course completely ceases and the vessel begins at once to dwindle, so that now the only venous supply of the liver is through the portal vein. The earliest appearance of the parietal system of veins is the formation of two short transverse veins (ducts of Cuvier) opening into the auricle on either side, which result from the union of an anterior cardinal, afterwards forming a jugular, vein, collecting blood from the head and neck, and a posterior cardinal vein which returns the blood from the Wolffian bodies, the vertebral column, and the parieties of the trunk. This arrangement persists throughout life in Fishes, but in Mammals the following transformations occur.

As the kidneys are developing a new vein appears (vena cava inferior), formed by the junction of their efferent veins. It receives branches from the legs (iliac) and increases rapidly in size as they grow further up it receives the hepatic veins, which by now have lost their original opening into the ductus venosus. The heart gradually descends into the thorax, causing the ducts of Cuvier to become oblique instead of transverse. As the fore-limbs develop, the subclavian veins are formed.

A transverse communicating trunk now unites the two ducts of Cuvier, and gradually increases, while the left duct of Cuvier becomes almost entirely obliterated (all its blood passing by the communicating trunk to the right side) (fig. 480, c. D). The right duct of Cuvier remains as the right innominate vein, while the communicating branch forms the left innominate. The remnant of the left duct of Cuvier generally remains as a fibrous band, running obliquely down to the coronary vein, which is really the proximal part of the left duct of Cuvier. In front of the root of

the left lung, another relic may be found in the form of the socalled vestigial fold of Marshall, which is a fold of pericardium running in the same direction.

In many of the lower mammals, such as the rat, the left ductus Cuvieri remains as a left superior cava.

Meanwhile, a transverse branch carries across most of the blood

Fig. 480.-Diagrams illustrating the development of the great veins. d c, ducts of Cuvier; j, jugular veins; h, hepatic veins; c, cardinal veins; s, subclavian vein; j i, internal jugular vein; j e, external jugular vein; az, azygos vein; e i, inferior vena cava ; r, renal veins; i, iliac veins; hij, hypogastric veins. (Gegenbaur.)

of the left posterior cardinal vein into the right; and by this union the great azygos vein is formed.

The upper portions of the left posterior cardinal vein remain as the left superior intercostal and vena azygos minor (fig. 480).

CIRCULATION OF BLOOD IN THE FOETUS.

The circulation of blood in the foetus differs considerably from that of the adult. It will be well, perhaps, to begin its description by tracing the course of the blood, which, after being carried out to the placenta by the two umbilical arteries, has returned, cleansed and replenished, to the foetus by the umbilical vein.

It is at first conveyed to the under surface of the liver, and

there the stream is divided,-a part of the blood passing straight on to the inferior vena cava, through a venous canal called the ductus venosus, while the remainder passes into the portal vein,

and reaches the inferior vena cava only after circulating through the liver. Whether, however, by the direct route through the ductus venosus or by the roundabout way through the liver,-all. the blood which is returned from the placenta by the umbilical vein reaches the inferior vena cava at last, and is carried by it