The bile that flows into the stomach has always appeared to me to be hepatic bile, from its light colour. I think that I have opened a sufficient number of living animals to convince me, that this bile is hardly ever found very green, and that it acquires this colour from the gall-bladder; and that it is this that is brought up by vomiting in some affections. The reflux of this bile appears to be an effect of the affection itself. This observation agrees with that made above, viz. that the hepatic bile alone flows into the duodenum in abstinence. It alone can then, as we may be convinced, flow into the stomach. During intestinal digestion, in which the cystic bile flows, it is evident, that the aliments going continually out of the pylorus, prevent it from passing there and entering the stomach; that which we find during fulness, was there then, or entered there before the peristaltic motion had begun to evacuate this organ.
When we open the gall-bladder in a dead body, we see that the bile there exhibits, according to its diseases, a variety of shades of colour, from that which is black as ink to a kind of transparent fluid. Ought we then to be astonished, if the vomitings in which the cystic bile is brought up, that has flowed into the stomach against the ordinary course of things, should contain matters of such various colours?
Development.
In the fœtus, the system of black abdominal blood is not insulated; it becomes a part of the two others, by means of the ductus venosus. There is then truly but one vascular system in the fœtus, whilst after birth, there are three separate ones, two with black blood and one with red.
In the fœtus, it is especially with the umbilical vein that the abdominal system with black blood is continued. The liver is a centre, in which both arrive from two different sides, and in which they unite, in a common trunk. The two columns of blood that they circulate, do not meet directly; their course forms a very remarkable angle.
When we examine attentively the orifice of the ductus venosus in the trunk, made by the union of these two veins, we see that it presents itself naturally to the blood of the umbilical vein; that that of the vena porta, on the contrary, cannot enter there. In fact, there is a little fold in the form of a valve, less evident, it is true, than many others, but yet existing. This fold is only a kind of projection, placed between the end of the vena porta and the ductus venosus, and which contracts the orifice of this, so that it is evidently narrower than the caliber of its own canal. The blood coming from the vena porta and passing at the side of this fold, presses it against the orifice, and thus forms an obstacle; that coming from the umbilical vein, falling, on the contrary, perpendicularly on this orifice, removes this fold, and enters the canal.
It hence follows that the ductus venosus is evidently destined to carry to the vena cava the residue of the blood of the umbilical vein; I say the residue; in fact, as this vein is very large and the ductus small in proportion to it, it is evident that the greatest part of the blood penetrates the liver, by the different ramifications that enter its substance.
The abdominal vascular system is less developed in the fœtus than afterwards; it consequently carries less blood to the liver; this is the same arrangement as in all the other veins. I would observe, that the small quantity, however, which the liver receives in this way, is more than compensated by that of the umbilical vein. This viscus is, then, habitually entered in the fœtus, by a greater quantity of fluid than at any of the other ages. Hence, 1st, why its nutrition is so developed and its size so great; 2d, why it is, in proportion to its size, heavier than in the after ages; 3d, why when we cut it in slices, there flows out a greater quantity of blood; 4th, why, when we dry slices of the liver of a fœtus, of the same thickness as others taken from the liver of an adult, and especially of an old person, they are reduced to a less size.
The disproportion of the size of the liver of the fœtus is more evident, the sooner it is examined after conception; this is the same as with the brain. As the fœtus advances towards birth, the liver approximates in its proportions to the other organs, that which it will have in the adult. From the observations of Portal, it is especially till the seventh month, that the liver is predominant. This circumstance appears to arise from this, that the umbilical vein transmits as much more blood in proportion to the fœtus, as it is less advanced in age.
At this age, the blood of the umbilical vein and that of the vena porta evidently mix, at least in a great measure, in the common trunk. Is their nature analogous? There is no experimental knowledge upon this point. But Baudelocque has many times observed that that of the umbilical vein is redder, and even approximates the nature of arterial blood. I have not accurately observed this fact in any animals except guinea pigs, in whom the want of transparency in the cord does not allow us to see a great difference in the blood of the arteries and of the umbilical vein; but this difference can be in fact more evident in man; now, in this case, the umbilical blood appears to lose this redness in the liver, for very certainly it is uniform beyond this viscus in the circulation of the fœtus, as I have often ascertained.