General Anatomy, Applied to Physiology and Medicine, Vol. 1 (of 3)

by Xavier Bichat

The first of these systems has, 1st, for origin all the capillaries of the abdomen, of the inferior extremities, and even those of the placenta; 2d, for common trunks, below the inferior vena cava, above the quadruple branch called the aorta; 3d, for agent of impulse the left side of the heart; 4th, for termination all the capillaries of the head and the superior parts. The second commences in these last capillaries, and is composed, 1st, for its trunks, of the superior vena cava and the descending aorta; 2d, for its agent of impulse, of the right side of the heart; 3d, for its termination, of the capillaries of the inferior parts.

The blood is then evidently divided in the first months after conception into two circulations, which cross, if it may be so said, in the form of the figure 8, as has been remarked by Sabatier; it is carried in each, from one assemblage of capillaries to another of the same vessels. Only instead of moving between the pulmonary capillary system and the general one, as in the adult, it moves between the superior and inferior part of this last system; we may then say in this point of view that the inferior and superior parts of the body are in opposition in the fœtus, as the lungs in the adult are in opposition to the rest of the body.

This complete opposition on the part of the circulation, between the upper and lower part of the body, in the first months of the fœtus, is probably the origin of the difference that takes place afterwards between these parts.

All physicians have observed this difference in diseases. If the median line frequently separates the affections of the right side from those of the left, the diaphragm seems often to be the boundary of many diseases. Who does not know, that scorbutic affections appear particularly below, that serous infiltrations are most frequent there, and that ulcers are infinitely more common in the inferior extremities, and that on the other hand, most cutaneous eruptions take place in the superior parts, &c.? Bordeu, who has said much of the division of the body into superior and inferior parts, considered one pulse as the precursor of evacuations from above, and another as that of those from below, he has however without doubt exaggerated this opposition of the two parts of the body; still it really exists, and I think that it is very probable, that the manner of the circulation of the fœtus is the primary source of it.

After the first months, things begin to change. The quantity of blood passing by the pulmonary artery was at first scarcely any thing, because the dilatation of the ductus arteriosus was so great, that it turned almost the whole of it into the descending aorta. This canal gradually contracting, the pulmonary arteries dilate, and then more blood goes through the lungs, and is brought by the pulmonary veins to the left auricle, which transmits it to the left ventricle, which sends it to the arch of the aorta; then the mechanism of the circulation described above begins to change, and approximate that of the infant, as we shall see.

Still this first mechanism predominates for a long time over the second; hence it happens that during the greatest part of the time that the infant is in the womb of the mother, it is the left ventricle that sends the blood to the superior parts, whilst the inferior receive theirs by the impulse of the right. Now as the parietes of the first are evidently thicker than those of the second, and the heart is further from the inferior than the superior parts, these last receive a stronger impulse than the others. This perhaps is a new source of the difference of the two halves of the body; hence nutrition is more active in that above, hence the degree of vital energy that it preserves a long time after birth, and which makes it susceptible, the head especially, of many more affections than the lower half.

As the period of birth approaches, the pulmonary artery sends more blood to the lungs, and less passes through the ductus arteriosus. For, as I have said, it is only in a gradual manner that the whole of this fluid, contained in the body, comes finally at birth to go through the lungs. Though before it undergoes no alteration there, it does not circulate the less, which is undoubtedly to habituate it to the passage that it is constantly to take after birth. The quantity of blood then is in a direct ratio to the age in the pulmonary artery, and in an inverse one in the ductus arteriosus.

This arrangement evidently requires a corresponding one in the foramen ovale; in fact, if in proportion as the ductus arteriosus contracted, this was not diminished also, all the blood would finally accumulate in the superior parts. For instead of passing from them to the inferior, the whole of it would return to them by the left auricle and the ventricle of the same side. In proportion as the ductus arteriosus is contracted, the foramen ovale being lessened also, the blood of the inferior vena cava, the whole of which cannot pass through there, begins to mix with that of the superior, enters the right auricle, then the right ventricle, afterwards returns by the lungs to the left auricle and ventricle and the aorta. What is the consequence of this? that this artery begins to receive from the left ventricle a much greater quantity of blood than can pass into the carotids and subclavians; a portion of it then goes into the descending trunk and is distributed to the inferior parts.

From what has been said, it appears, that the two portions of the blood of the fœtus are almost wholly separate in the first months; all that comes from the inferior vena cava goes into the ascending aorta; all from the superior passes into the descending, the lungs receiving scarcely any except by the bronchial arteries for their nutrition. But as the period of birth approaches, these two portions of blood begin to mix, and the circulation then has an arrangement between that of the adult and that of the first months. At birth even, the foramen ovale and the ductus arteriosus are much contracted, the circulation goes on in the mother's womb almost in the same way as it does after birth; the whole difference is that the fluid is of the same nature, because respiration has not taken place. The sudden change of the circulation at birth, arises particularly from the introduction of red blood into the economy. As to the mechanical phenomena, they are gradually produced by the gradual contraction of the two openings of communication. The blood gradually ceases to move from the inferior to the superior capillaries; it then goes from both of these to those of the lungs and reciprocally.

In considering the circulating phenomena, we err in supposing that their change is sudden at birth. It is sufficient to examine the foramen ovale and the ductus arteriosus at different periods of pregnancy, to see that they contract successively, and that consequently these phenomena are successive, so that if the fœtus should remain in the womb a long time beyond its period, and the contraction should continue in the foramen ovale and the ductus arteriosus, the blood would circulate as in the adult, from the pulmonary to the general capillary system exclusively, and reciprocally. The only difference would be in the uniformity of its colour, because it would pass through the first system, without coming in contact with the air.