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At one time it was supposed, that a direct communication exists between the maternal and foetal vessels, but this notion has been long exploded. We have the most decisive evidence, that the connexion is of the most indirect nature. Wrisberg made several experiments, which showed that the fluid of the foetal circulation is not drained when the mother dies from hemorrhage. It has been shown, too, that if the uterine arteries be injected, the matter of the injection passes into the uterine veins after having been effused into the lobes of the placenta, and the same thing happens when the uterine veins are injected. If, on the other hand, the injection be thrown into the umbilical arteries or vein, the matter passes from one of these sets of vessels into the other, is effused into the fœtal side of the placenta, but does not pass into the uterine vessels. When, however, an odorous substance, like camphor, is injected into the maternal veins of an animal the foetal blood ultimately assumes a camphorated odour. Magendie injected this substance into the veins of a gravid bitch, and extracted a foetus from the uterus, at the expiration of three or four minutes: the blood did not exhibit the slightest odour of camphor; whilst that of a second foetus, extracted at the end of a quarter of an hour, had a decidedly camphorated smell. This was the case, also, with the other fœtuses. Such communication may, however, have been owing to the same kind of transudation and imbibition, of which we have spoken under the head of absorption, and may consequently be regarded as entirely adventitious; and the fact of the length of time, required for the detection of the odorous substance, favours this idea; for if a communication, of even an indirect nature, existed between the mother and the foetus, the transmission ought certainly to have been effected more speedily.
The transmission of substances from the fœtal to the maternal placenta is yet more difficult. Magendie was never able to affect the mother by poisons injected into the umbilical arteries and directed towards the placenta; and he remarks, in confirmation of the results of the experiments of Wrisberg, that if the mother dies of hemorrhage, the vessels of the foetus remain filled with blood.
Another fact, that proves the indirect nature of the connexion which exists between the parent and child, is the total want of correspondence between the circulation of the foetus and that of the mother. By applying the stethoscope to the abdomen of a pregnant female, the beating of the heart is observed to be twice as frequent as that of the mother. Again, examples have occurred in which the foetus has been extruded with the placenta and membranes entire. In a case of this kind, which occurred to Wrisberg, the circulation continued for nine minutes; in one, described by Osiander, for fifteen minutes; in some, by Professor Chapman, for from ten to twenty minutes; and in one by Professor Channing, of Boston, and Dr. Selby, of Tennessee, where a bath of tepid water was used to resuscitate the foetus, for an hour. In other cases of a simi
lar kind, where the child could scarcely breathe and was in danger of perishing, the life of the placenta has been maintained by keeping it in water of a temperature nearly equal to that of the body, and the child has been saved. All these facts prove demonstratively, that the foetus carries on a circulation independently of that of the mother, and that whatever passes between the foetal and maternal vessels is probably exhaled from the one and absorbed by the other, as the case may be. The fluid sent to the fœtus is supposed by some, indeed by most physiologists,—to be the maternal blood. Schreger, however, maintains that the communication of any nutritious fluid from the mother to the foetus and vice versa takes place by means of lymphatics, and not by blood-vessels; and that the maternal vessels exhale into the spongy tissue of the placenta the serous part of the blood, which is taken up by the lymphatics of the foetal portion, and conveyed into the thoracic duct.
The facts, previously brought forward, show, that the foetus may be developed without any umbilical cord or umbilicus; and those we have just detailed, exhibit, that the foetal circulation is at all events largely independent of that of the mother; whilst the position, that the placenta is the medium by which nutritive matter of any kind passes from the mother to the fœtus and vice versa rests upon singularly feeble and inadequate evidence. The functions, which it appears to execute, will engage attention presently.
Lastly, Lobstein and Meckel suppose, that the gelatinous substance of the cord is one of the materials of foetal nutrition. This opinion they found on the circumstance of the albuminous nature of the substance, and the great size which it gives to the cord at the early periods of foetal life, as well as on the great development of the absorbent system of the foetus, proceeding from the umbilicus to the anterior mediastinum.
All these speculations regarding the various sources of nutritive. matter are sufficient evidence of the uncertainty that prevails on this interesting topic; yet Magendie affirms, "that it appears certain, that the placenta imbibes from the mother the materials necessary for the development of the organs!"
Some of the most recent writers on the subject are of opinion, that the sources whence the nutritive matter is derived varies at different periods of gestation. Lobstein, for example, thinks, that the venous radicles of the rudimental placenta obtain nutritious fluids from the mother, prior to the formation of the arteries; but that afterwards all circulation between the uterus and placenta ceases; and the umbilical vesicle, (a small body of the size of a large pea seated, as we have seen, in the umbilical cord between the chorion and amnion, and supposed to communicate with the intestine, and to contain a nutritive fluid,—but of the anatomy of which we know but little,) the liquor amnii, and the jelly of the cord are the nutritive materials. Meckel considers, that the placenta is in no case
the source of nutritive matter. He regards it as an organ for the aeration of the blood; whilst nutrition is effected by the fluid of the umbilical vesicle, at the commencement; by the liquor amnii till mid-term; and by the jelly of the cord for the remainder of gestation. Lastly, according to Beclard, nutrition is accomplished, during the first weeks, by the fluid of the umbilical vesicle; afterwards by the liquor amnii, and the jelly of the cord; and, as soon as the plaoenta is formed, by that organ.
It is manifest, however, that we cannot regard as nutritive matters those substances that are secreted by the foetus itself. It is impossible, that any development could occur without the reception of materials from without. We have seen, that when the ovum passes from the ovarium to the uterus, it contains within it a molecule and fluids, which are probably destined for the nutrition of the new being, and which afford the necessary pabulum for the increase, that occurs between impregnation and the period at which an adhesion is formed between the ovum and the inner surface of the uterus. The mother, having furnished the nutritive material. in the ovum, she must continue to provide it in the uterus; and so soon as a vascular communication is formed between the exterior of the ovum and the interior of the uterus, nutritive elements are doubtless received by the embryo;—for otherwise it would perish from inanition. What then can be the nature of these elements? Do they consist of blood, which is laid hold of by the foetus at this early period, when no circulatory system is apparent; or are the blood-vessels distributed to the membranes of the ovum, to enable them to continue the secretion of that nutritive matter, which they took with them from the ovarium, and which must necessarily have had a maternal origin? The latter certainly is the more probable supposition, and it is a strong argument in favour of the amnion being supplied with blood from the uterus, rather than from the fœtus; for, if we admit it to be in any manner inservient to nutrition, its production must be extraneous to the body which it has to nourish. These observations apply equally to the jelly of the umbilical cord, which is probably secreted by the membranous envelopes, and may consequently be regarded as a nutritive material derived from the parent.
On the whole, therefore, it appears at least doubtful, whether the fœtus receives from the mother any nutritive fluid through the placenta; whilst there is strong reason for believing, that, from the very earliest period of foetal formation to the last, it is nourished on secretions formed at the expense of the mother, and that these are, essentially, the liquor amnii and the jelly of the cord.
If we admit this, however, it is obvious, that the nutritive fluid, when received into the system will have to be formed into blood by the action of the foetus, in a manner, bearing some analogy to what occurs in the adult, or in the simplest of living beings, in which
the nutritive fluid is absorbed at the surface of the body. Of the mode in which such conversion is effected we are in the same darkness, that envelopes all the mysterious processes which are esteemed organic and vital; but that the foetus is capable of effecting it we have irrefragable proof in the oviparous animal, where there can be no communication, after the egg is laid, between the embryo and the parent. Yet we find it forming its own blood from the yolk surrounding it, and undergoing its full and regular development from causes seated in itself alone.
Of those physiologists who consider that the mother sends her blood to the placenta, to be taken up by the fœtal vessels, all do not conceive, that it is in a state adapted for the nutrition of the new being: some are of opinion that the placenta or the liver, or both, modify it, but in a manner which they do not attempt to explain. In favour of such an action being exerted by the placenta, they state that it is clearly the organ which absorbs the fluid, and that every organ of absorption is necessarily one of elaboration;—a principle which we have elsewhere proved to be unfounded; and, moreover, that the blood, conveyed to the foetus by the umbilical vein, differs essentially in colour from that conveyed to it by the umbilical arteries,—a fact, which we shall see, can be accounted for more satisfactorily. In support of the view, that a second change is effected in the liver, they affirm, that a great part of the foetal blood ramifies in the substance of that organ before it reaches the heart; a part only going by the ductus venosus; and that the great size of the liver, during fœtal life, when its function of secreting bile can be but sparingly exerted, is in favour of this notion.
The opinion, that some change is effected upon the blood in the liver, is certainly much more philosophical and probable than the belief of Haller, that the object of its passage through that organ is to deaden the force with which the mother projects the fluid into the fœtal vessels. We have seen, that it is extremely doubtful, whether she transmits any; and that if she does, the communication is extremely indirect.
M. Geoffroy Saint-Hilaire appears also to think, that the blood of the mother, which he conceives to be sent through the placenta to the foetus, is unfitted for foetal life, before it has undergone certain modifications. The blood, according to him, which leaves the placenta, proceeds in part to the liver and the remainder to the heart. In the liver it forms the material of the biliary secretion, or at least of a fluid, which, when discharged into the intestines, irritates them, and provokes a copious secretion from the mucous or lining membrane. This mucus, according to M. SAINTHILAIRE, is always met with in the stomach and intestines of the fœtus; whilst the presence of meconium, and of other excrementitious matters in the intestines, shows, that digestion must have taken place. This digestion he considers to be effected upon the mucus,
secreted in the manner just mentioned; and, in support of its being inservient to sanguification, he affirms, that its quantity is too great for the simple purpose of lubricating the parts; that mucus is the first stage of all organic compounds; that it predominates in all young beings; is the foundation of every organ; more capable of assimilation than any other substance, &c. But independently of the whole of this view being entirely hypothetical, it cannot be esteemed probable, that the fœtus is nourished by one of its own secretions. All secretions must be formed from blood. Blood must, therefore, preexist in the foetal vessels, and the process, indicated by SAINTHILAIRE, be unnecessary.
Allusion has already been made to the opinions of Schreger, on the nutrition of the fœtus. These were developed in a letter written by him, in 1799, to Soemmering. He considers, that all communication of nutritious matter between the mother and fœtus occurs through the lymphatics, which he has described as existing in considerable numbers in the placenta and umbilical cord. The red blood, flowing in the maternal vessels, is too highly charged with carbon, and with other heterogeneous substances, he thinks, to serve for the nutrition of the foetus. Its serous part, which is purer and more oxygenized, is therefore alone exhaled. The uterine arteries pour this serum into the spongy texture of the placenta, whence it is taken up by the lymphatics of the fœtal portion. These convey it along the umbilical cord to the thoracic duct, whence it passes into the left subclavian, vena cava superior, right auricle and ventricle, ductus arteriosus, aorta; and, by the umbilical arteries, is returned to the placenta. In this course, it is mixed with the blood, and becomes itself converted into that When it attains the placenta, the blood is not poured into the cells of that organ, to be transported to the mother, but it passes into the umbilical vein, the radicles of which are continuous with the final ramifications of the umbilical arteries. Lateral pores, however, exist in the latter, which suffer fluids to escape, that cannot be elaborated by the foetus, or which require to be again submitted to the maternal organs, before they are fitted for its support. These fluids, according to Schreger, are not absorbed by the veins of the uterus, but by the lymphatics of that viscus, which are so apparent in the pregnant state and have been injected by Cruikshank, Meckel, &c. In his view, therefore, the conversion of the serous fluid into blood, is chiefly effected in the lymphatic system, and it has been a favourite hypothesis with many physiologists, that those organs, regarding whose functions we are so profoundly ignorant, and whose development is so much greater during intrauterine than extra-uterine existence,—as the thymus, and thyroid glands, and the supra-renal capsules,—are, in some way, connected with the lymphosis or hæmatosis of the fœtus.
We have already referred to the conjectures, that these organs