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are diverticula for the blood of those parts, the functions of which are not exerted until an after period of existence. Broussais makes the thyroid a diverticulum to the larynx; the thymus a diverticulum to the lungs, and the supra-renal capsules to the kidneys. Notwithstanding these ingenious speculations, however, our darkness, with regard to the true functions of these singular organs, is not the less impenetrable.

To conclude. The most plausible opinion, that we can form on this intricate subject is, that the mother secretes the substances, which are placed in contact with the fœtus, in a condition best adapted for its nutrition; that in this state they are received into the system, by absorption, as the chyle or the lymph is received in the adult,—undergoing modifications in their passage through the fœtal placenta, as well as in every part of the system, where the elements of the blood must escape for the formation of the various tissues.

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With regard to the precise nutritive functions, executed in the foetal state, and first as concerns digestion, it is obvious, that this cannot take place to any extent, otherwise excrementitious matter would have to be thrown out, which, by entering the liquor amnii, would be fatal to many important functions, and probably to the very existence of the fœtus. Yet that some digestion is effected is manifest from the presence of meconium in the intestines, which is probably the excrementitious matter, arising from the digestion of the mucous secretions of the alimentary canal.

Respiration, as accomplished by lungs, does not exist; and we have already seen, that the idea of the fœtus possessing the kind of respiration of the aquatic animal is inadmissible. An analogous function to the respiration of the adult however occurs, as respects the changes effected upon the blood. bable, that the blood is sent to the placenta to be aerated there, as it is in the lungs in extra-uterine existence. Such was the opinion of Sir Edward Hulse, of Girtanner, Stein, and we may say, such is the opinion of many of the most enlightened physiologists of the present day. The chief arguments, adduced in support of this opinion, are,—the absolute necessity for air to every living being, animal or vegetable; the no less necessity, for a free circulation of blood along the umbilical cord to the placenta, to the life of the fœtus; the analogy of birds, in which the umbilical vessels are inservient to respiration by receiving the external air through the pores of the shell, so that if the shell be covered with varnish, respiration is prevented and the chick dies.

The sensible evidences of these changes being effected by the placenta are not like those, which we possess regarding the aeration of the blood in its passage through the lungs of the adult, where the venous, differs so essentially from the arterial blood. It is indeed asserted, in works of anatomy, that "the effete blood of the

umbilical arteries becomes regenerated in the placenta, assumes a brighter hue, and is returned to the fœtus by the umbilical vein," but this is not in accordance with experiment and observation. Bichat made numerous dissections of young pigs whilst still in utero, and he uniformly found the blood of the arteries and veins presenting the same appearance and resembling the venous blood of the mother. Not the slightest difference was observed between the blood of the aorta and that of the vena cava; nor between that of the carotid artery and of the jugular vein. He made the same observations in three experiments of a similar kind on the fœtuses of the dog. He also frequently examined human foetuses that had died in utero, and always found the same uniformity between the arterial and venous blood: hence he concludes, that there is no difference between the arterial and the venous blood of the fœtus, at least in appearance. Similar experiments by Autenrieth furnished the same results. It is important to bear this fact in mind, inasmuch as it may be received as one of the evidences that a foetus has not respired.

The apparent identity, however, between the blood passing to the placenta by the umbilical arteries and that returning by the cord is not real. The slightest reflection will show, that they must be essentially different. It is from the blood, carried by the umbilical vein and distributed over the body, that all the organs of the fœtus have to derive the materials of their nutrition and development; and being deprived of these materials the fluid must necessarily be different in the umbilical arteries from what it is in the urnbilical vein. The researches of more modern chemistry have not been directed to the foetal blood, but Fourcroy analyzed it and found it differ materially from the blood of the child that had respired. He asserts, that its colouring matter is darker, and seems to be more abundant; that it is destitute of fibrine and of phosphoric salts, and is incapable of becoming florid by exposure to the influence of atmospheric air. Under the head of circulation it was remarked, that the coloration of the blood is perhaps of no farther importance than as indicating, that the vital change of aeration has token place in the lungs. In this case, we have the vital change effected without any such colora

Yet how, it may be asked, is the modification in the blood produced where no placenta and no umbilical cord exists? And can we suppose that in such cases the aeration is effected by the liquor amnii containing an unusual quantity of oxygen, as has been presumed by some physiologists? These are embarrassing questions—more easily propounded than answered.

It is in the fœtal circulation that we observe the most striking peculiarities of intra-uterine existence. Of its condition at the very earliest periods we know little that is not conjectural. We will, therefore, consider it as it is effected during the last months of utero-gestation. From the sketch already given of the circulatory

organs of the fœtus, it will be recollected, 1st, that the two auricles of the heart communicate by an aperture in the septum, called the foramen ovale, which has a valve opening towards the left ventricle; 2dly, that near the orifice of the vena cava inferior is the valve of Eustachius, so situated as to direct the blood of the cava into the foramen ovale; 3dly, that the pulmonary artery has a vessel passing from it into the aorta,—the ductus arteriosus; 4thly, that two arteries, called umbilical, proceed from the internal iliacs to the umbilicus and placenta; and, lastly, that the umbilical vein from the placenta pours part of its blood into the vena porta; and a part passes by the ductus venosus,—a foetal vessel,into the inferior cava.

The course of the circulation, then, is as follows:- The blood of the umbilical vein,—the radicles of which communicate with those of the umbilical arteries in the placenta,—proceeds along this vein to the umbilicus, and thence to the liver. A part of this traverses the ductus venosus, enters the vena cava inferior, and becomes mixed with the blood from the lower parts of the fœtus; the remainder passes into the vena porta, is distributed through the liver, and, by means of the hepatic veins, is likewise poured into the vena cava. In this manner it attains the right auricle. Owing to the arrangement of the valve of Eustachius, the blood passes immediately through the foramen ovale into the left auricle,without being mixed with the fluid proceeding from the upper parts of the body into the right auricle through the vena cava superior. The left auricle is consequently as much developed as the right, which it would scarcely be, did it receive only the blood from the lungs. Were it not as large, it is obvious, that it would be insufficient to carry on the circulation, when the whole of the blood passes through the lungs, and is poured into it after respiration is established.

The above are the opinions of Wolf and Sabatier regarding the use of the Eustachian valve. According to this view, if the valve did not exist, the aerated blood, conveyed to the heart by the ductus venosus, instead of being directed into the left auricle through the foramen ovale, would pass into the right auricle, and thence,—in part, at least,—into the right ventricle; from which it would be transmitted, through the pulmonary artery and ductus arteriosus, into the descending aorta; so that no part of the body, above the opening of the duct into the aorta, could receive the aerated blood, whilst much of that, which passed along the aorta, would be returned to the placenta by the umbilical arteries. But as the blood is directed into the left auricle by the Eustachian valve, it passes from thence into the left ventricle, and is forced by it into the aorta, which distributes it to every part of the system, and thus conveys the regenerated fluid to every organ.

Dr. Wistar has also suggested, that, without this arrangement of the Eustachian valve, the coronary arteries, distributed to the heart, VOL. II.

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would be unfit for supporting the life of that organ, inasmuch as they would be deprived of a regular supply of revivified blood.

From the left auricle, the blood passes into the left ventricle, and from the left ventricle into the ascending aorta and to the upper parts of the body, from which it is brought back, by the vena cava superior, into the right auricle; thence it is transmitted into the right ventricle, and, by the contraction of the ventricle, into the pulmonary artery. By this vessel it is sent, the greater part through the ductus arteriosus into the descending aorta, and a small part to the lungs. From the lungs it is returned into the left auricle by the pulmonary veins. Through the descending aorta the blood, conveyed in part by the ductus arteriosus, and in part by the contraction of the left ventricle, is distributed, partly to the lower extremities, from which it is returned by corresponding veins into the vena cava inferior, and partly by the umbilical arteries to the placenta.

This view of the circulation supposes what is disputed,that the blood of the vena cava superior and of the vena cava inferior does not undergo admixture in the right auricle; whence it would follow, that some parts of the body receive a purer blood than others,—the upper parts, as the head and neck, receiving that which flows immediately from the placenta, whilst the lower parts do not obtain it until it has circulated through the upper. Under any view it is manifest, that it is not the whole of the blood, which is distributed to the organ of aeration, as in the adult, but a part only as in the batracia.

Bichat and Magendie contest, and we think successfully, the explanation of Wolf and Sabatier, regarding the use of the valve of Eustachius and the non-admixture of the blood of the two cavæ in the right auricle. In their opinion, the two bloods do commingle; but, owing to the existence of the foramen ovale and the arrangement of the valve of Eustachius, the left auricle is filled simultaneously with the right; and, consequently, the same kind of blood must be distributed to both the upper and lower portions of the body. The uses of the foramen ovale and ductus arteriosus are explained as follows. As the left auricle receives but little blood from the lungs, it could furnish but a small quantity to the left ventricle, did it not receive blood through the foramen ovale; and again, as the lung is exerting no function, during the state of fœtal life, the blood is sent along

the pulmonary artery and ductus arteriosus into the aorta, so that the contraction of both ventricles is employed in propelling the blood along the aorta to the lower parts of the body and to the placenta. Without this union of forces it is conceived, that the blood could not be urged forward as far as the placenta.

After all, then, the great difference between the fœtal and adult circulation is,—that, in the former, a part of the blood only proceeds to the organ of sanguification; that the aerated blood is

poured into the right auricle instead of the left; that, instead of proceeding through the lungs, a part of the blood gets at once to the left side of the heart, and the remainder goes directly from the pulmonary artery into the aorta ; that a part of the aortic blood proceeds to the lower extremities, and the remainder goes to the placenta, from which it is returned into the inferior cava.

With regard to the nutrition, (properly so called,) of the fœtus, it is doubtless effected in the same manner as in the adult; and our ignorance of the precise nature of the mysterious process is equally great. During the whole of fœtal existence it is energetically exerted, and especially during the earlier periods. Soemmering has asserted, that the growth of the fœtus fluctuates; that in the first month it is greatest; in the second, less; in the third, greater; less, again, in the fourth; and then greater until the sixth, when it diminishes until birth.

There are one or two singular circumstances, connected with the nutrition of the fœtus, which cannot be passed over without a slight notice.

Owing to inappreciable causes, the different parts of the fœtus, or some particular part, may be preternaturally developed or be defective, giving rise to what have been termed monstrosities. Three kinds of monsters may be considered to exist. The first comprises such as are born with an excess of parts, as with two heads on one trunk, two trunks to one head; with four arms and four legs; twins with a band uniting them, as in the case of the Siamese twins, &c. The second includes those in which parts are defective, as acephali, anencephali, &c.; and the third, those in which there is deviation of parts, as where the heart is on the right side, the liver on the left, &c.; where, in other words, there is transposition of the viscera.

The hypotheses, that have been advanced to account for these formations, as well as for those in which the parts are irregularly developed, may be reduced to three; the others, that have been indulged, having no probability in their favour. First. They have been attributed to the influence of the imagination of the mother over the foetus in utero. Secondly. To accidental changes, experienced by the fœtus at some period of uterine existence; and Thirdly. To some original defect in the germs.

The first of these causes has been a subject of keen controversy amongst physiologists, at all periods. We have seen, that the mother transmits to the fœtus the materials for its nutrition; and that, to a certain extent, the nutrition is influenced by the character of the materials transmitted; so that if these be not of good quality or in due quantity, the fœtus will be imperfectly nourished, and may even perish. Any violent mental emotion may thus destroy the child, by modifying the quantity or quality of the nutritive matter sent to it. Small-pox, measles, and other contagious diseases can also be unquestionably communicated to the fœtus in utero; so

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