that the portion of allantois which is thus constringed within the navel and the umbilical cord becomes a mere tube. This allantoidal tube is the urachus; the portion of the sac remaining in the belly becomes urinary bladder; while all the rest of it which lies in the ovum between the amnion and the chorion is allantois. Air blown into the bladder then ought, after distending it, to run in the tube of the urachus, along the navel string, after passing through which it should proceed to inflate the true allantois.

Very clear descriptions of the allantois may be seen in M. Flourens' lectures on the generation of mammiferous animals, and in Prof. Coste's great work on the development of organized bodies. Both of those writers have illustrated the subject with engravings, those of Prof. Coste being worthy to be called truly admirable.

Umbilical Vesicle.-The human yelk, as I said, is a microscopic globule filled with vitellary corpuscles. When the blastoderm has partly undergone the morphological changes that convert it into the earliest rudimental embryon, part of the yelk corpuscles still remain unappropriated; and, as they are still contained in their original vitelline membrane, they constitute a small but visible ball called the umbilical vesicle. Originally, the vitellus was a sphere, as in the Figure 57 annexed.

The blastoderm is developed upon a segment of this sphere, as in

Fig. 58.

Fig. 59.

Fig. 60.

When the blastoderm doubles or folds its edges inwards, it pinches a portion of the vitellary ball, as in Fig. 59.

In a still further progress, Fig. 60, the portion of the vitellary ball that remains outside of the embryon is connected to the embryo by a delicate tube or vitellary duct. This tube or duct opens into the intestinum ileum of the embryo. Velpeau says that the yelk matter contained in the umbilical vesicle can be pressed along the tube and through it squeezed into the gut. It is supposed to furnish a pabulum to the early embryo, but is lost after the fourth month; for, by that time, the amnion has grown so considerably as to fill up completely the cavity of the chorion: Hence the umbilical vesicle, being squeezed flat betwixt the amnion and chorion, finally disappears, becoming of no functional value when the child has completely established its branchio-absorptive connection with the parent-just as happens to the urachus and allantois.

Omphalo-Mesenteric Vessels and Cord.-In perfect ova, aborted at the period of two months, or a little later, the Student will readily distinguish the umbilical vesicle shining through the

h

Fig. 61.

chorion and lying betwixt it and the delicate amniotic membrane. I add here a figure (61), that may serve to explain its arrangement. Let a be a portion of the abdomen of the embryo, and cc the navel or umbilical ring; bb the navel string or cord laid open; d the umbilical vein bringing back the blood from the placenta, and passing into the belly at the ring to go to the liver; e f the two umbilical arteries of the foetus; h the umbilical vesicle or vitelline sac, whose pipe, conduit, or efferent duct runs along the umbili cal cord to the navel, and passing into the belly empties itself in the ileum g g, which bends up to receive the discharge; k l represents the omphalo-mesenteric vessels.

In very early states, the knuckle of ilium rises quite high up in the root of the umbilical cord-occasionally it becomes fixed there, and the child, continuing to grow, is at length born with an irreducible exomphalos. A careless accoucheur may, in cutting the navel string, have the misfortune to cut off the top of the arc of intestine, and thus subject the miserable neonatus to the disgusting inconvenience of an artificial anus, as happened in a case within my knowledge. I have seen the major part of the convolutions of the small intestines detained in an immense exomphalic tumor, covered only by the cord and a lining of peritoneum to which they irreducibly adhered. As the cord is essentially deciduous, no hope is left to save a child thus deformed. It is non-viable. Fig. 62 shows one of these cases that fell under my care a few years since.

Now, as the umbilical cord is lined externally with amnion, it is clear that the umbilical vesicle

lies outside of the amnion and inside of the chorion within a space which, perhaps, might be properly called the allantoidal space. There is no doubt of the normal existence of this allantois or allantoides for the birds and the mammiferous quadrupeds; but as to man it is much questioned, and, as I have said above, it cannot be demonstrated that there is a sac that may be dissected out, existing betwixt the amnion and chorion. Noortwyck's fine dissertation upon it towards the end of his volume," Uteri Humani Gravidi Anatome et Historia," 4to. 1743, appears to me to settle the question: in his criticism on Walter

Fig. 62.

Needham's views of the sac, Noortwyck shows that it is indifferent whether there be a sac or no, for the space between the chorion and amnion is to all intents and purposes a true and sufficient allantois, one in which the urine of the early foetus can be discharged, as it is well known to be in the allantois of the bird, in which urinous concretions may be found.

After all, the Student may rest satisfied upon the point, in so far as to understand that an allantois is a urinary bladder constricted in the middle, like an hour-glass; the narrow neck being the urachus; the interior sac being the ordinary bladder of urine, and the one lying betwixt the chorion and amnion the real allantois.

The embryo has now established its connection with the parent; it has surrounded itself with its amniotic membrane, which fills with the liquor amnii in which the new being is suspended.

As its umbilical cord comes out of the abdomen nearest the pelvic extremity of the embryo, it hangs suspended with its head downwards whenever the woman is in a sitting or standing posture. It is true that the cord lengthens daily, and sometimes attains the length of six feet, although inclosed in a womb never more than twelve inches long. With such a great length, or even with a cord of eighteen inches long, it can no longer be said to be suspended; still the cephalic extremity of it falls to the lowest place, and the foetus as well as the embryo directs its head to the os uteri-it presents its head to the os uteri during the utero-gestation as well as in labor.

Circulation of the Fœtus.-The circulation of the foetus is peculiar to it, and its continuance in the same way after birth is inconsistent with respiratory life. If, therefore, the foetal circulation does not give place to the respiratory circulation, the neonatus perishes. This often happens. It is equally true, on the other hand, that, if the foetal characteristics of the circulation are lost before its birth, the child must of necessity be born dead.

Let us inquire into the nature of the foetal circulation.

The heart of the child in utero has four cavities-viz: a right and a left auricle, and a right and a left ventricle.

An opening in the septum auricularum, which is called the foramen ovale, or Botalli's foramen, and which, on the left side of the septum, is covered by a light floating valve, the valve of the foramen of Botalli, virtually converts these two chambers into one, just as two apartments are thrown in one by opening a wide door between them.

The left ventricle gives origin to the aorta. The right ventricle gives origin to the pulmonary artery. But, to speak rigorously, the pulmonary artery does not exist in the very beginning; for that which is called pulmonary artery is, in truth, the ductus arteriosus, from which the pulmonary artery afterwards arises at a more advanced period of the gestation. Seeing that this is the case, and that the ductus arteriosus joins the aorta below the arch, it is apparent that, when the right and left ventricles contract simultaneously, they con

cur, by their united power, to drive the blood along the tube of the aorta; and this combination of the force of both the ventricles is, perhaps, requisite to propel, not only the blood that circulates within the limits of the child's body, but also that which it sends far beyond those limits, to take up plasma and oxygen in the placental tufts, at the distance, sometimes, of six feet, and generally not less than twenty-four inches from the systolic source. Thus it is seen that the foetal heart, though divided like that of the breathing warm-blooded mammal, into four distinct chambers, is, by means of the foramen of Botalli and the ductus arteriosus, reduced back, in fact, to the condition of the fish's heart, which has but two cavities, one auricle and one ventricle, while the placenta, which is its branchial organ of aëration, takes up, like the gills or branchia of the fish, the oxygen it finds in the medium wherein it exists. Thus the heart employs the strength of both its ventricles to carry on such an enormous circulation.

There can nowhere be discovered a more admirable adaptation of a simple machinery to produce compound results, than in that of the foetal circulation; for, by the arrangement above mentioned, the single tube of the aorta is capable of effecting the double purpose of conducting the aërated blood to the tissues to oxygenate them; and, at the same time, of carrying back the carbonated blood to the placenta. The aorta, in this sense, is at once an oxygeniferous and a carboniferous tube. For, be it understood, the blood, when endowed with oxygen in the placenta, returns along the umbilical vein to the navel, and, running at the edge of the falciform ligament of the liver, enters the great fissure, and divides; part of the fluid entering into the hepatic porta, the rest continuing its course through the ductus venosus, is delivered into the left hepatic vein, which pours it into the lower cava. From the cava, it enters the lower, right, posterior part of the right auricle, behind the curtain-like valve of Eustachi, which conducts it across the posterior part of the auricle to the foramen of Botalli. Here, the current lifts the valve on the left side of the septum auricularum, to fill the left auricle. The auricle, being full, contracts, and pushes it into the left ventricle, whose next contraction injects it into the aorta: thus the oxygenated blood of the placenta reaches the aorta. Much of it is now determined to the brain and the superior extremities; the rest, turning through the aortic arch, is distributed in all the branches of that great trunk, a portion going back to the placenta again, in common with the carbonated blood of the fœtus. This is the systemic circulation of the foetus.

But that portion of the placental blood which passes into the carotids and subclavians, gives up, in their capillaries, its oxygen and part