Respecting the laws which regulate the forcible expulsion of these gases from the stomach or intestines, little need here be said. Though greatly influenced by habit, still the act is essentially voluntary. Its mecha nism is so closely akin to that of defwcation as not to require any separate notice. Whether the immediate stimulus to this expulsive act is always mere intestinal distention, or whether it is sometimes determined by the quality (as well as quantity) of the elastic fluids, cannot at present be decided.
We are equally ignorant as to how far, fail ing such an expulsion, these gases are capable of being absorbed into the blood ; and if so, where they emerge from the vascular system, or what form they assume in doing so. The small quantity of sulphuretted hydrogen really present in the most offensive flatus, and the comparative harmlessness of carburetted hy drogen in the proportions in which it would be dissolved by the blood, prohibit us from coming to any conclusion based on the ordi nary physiological action of these two gases. We can but conjecture, that whatever ab sorption they may undergo is slow enough to allow of the much quicker destruction of their poisonous properties by a more or less perfect oxidation.
Arteries of the intestines.— We have seen that the stomach and duodenum are supplied with arterial blood by means of various twigs derived from the three branches of the cceliac axis, which springs from the upper part of the abdominal aorta. The remainder of the intestinal canal is furnished with arteries which are given off by two large branches of the abdominal aorta. These branches are named, from their position and distribution, the superior and the inferior mesenteric.
The superior mesenteric artery (a,fig. 277.), the longer of these two branches, is distributed over that large seg,ment of the intestine which is formed by the lower part of the duodenum, the whole of the jejunum, ileum, and ccecum, and the first two-thirds of the colon. The trunk of the vessel comes off from the aorta, at a point which about corresponds to the upper border of the second lumbar vertebra. It is separated from the cceliac axis by the pancreas ; and hence is distant about a third of an inch from the origin of the latter vessel. From this commencement, it passes down wards and forwards, crossing over the termi nation of the duodenum, so as to reach the upper part of the mesentery. It now con tinues downwards between the two layers of this fold of peritoneum, uhich it occupies near its attachment to the posterior wall of the abdomen. Hence its length and direction
correspond to those of the attached border of the mesentery itself ; and are such, as to conduct it downwards and obliquely towards the left side, to a termination that corresponds to the end of the ileum, or the commencement of the ccecum. But the branches given off to these latter segments, of the intestine by the trunk of the vessel are so large, and so directly continuous with its previous course, that it is only in a very arbitrary and limited sense that we can speak of it as ending in this situation.
The arrangement of the larger or primary branches of the superior mesenteric artery is liable to great variation, but is generally as follows.
The trunk of the superior mesenteric artery is directly continuous with a large vessel (b, fig. 277.), which, when it has reached a dis tance of about two inches from the ccecum, divides into two others ; of these the upper (d,fig. 277.)passes towards the ccecum, and the lower (c,fig. 277.) towards the ileum. The ileo colic artery (b,fig. 277.), as the common trunk is named prior to its bifurcation, usually gives off from its right side one of rather smaller size, about three inches from the border of the bowel. The latter, which is called the arteria colica dextra, or right colic artery (e, fig. 277.), often arises by a separate trunk from the superior mesenteric. It takes a course almost horizon tally outwards, or towards the right side, lying underneath the single layer of perito neum which covers in the ascending colon, so as to reach this part of the large intestine at or near the middle of its height. Finally, at a dis tance of little more than an inch from its entering the mesentery, the trunk of the supe rior mesenteric artery gives off a large branch, the arteria colica media (f,fig. 277.), which passes upwards and backwards, enters between the two layers of the transverse meso-colon, and is distributed to the transverse colon, which it reaches at the middle of its posterior border. Besides these named branches, the superior mesenteric gives off numerous arte ries (at g, fig,. 277.), of almost equal size, which have not received any special designa tion. These twenty or thirty branches leave the left side of the artery, at various points between the lower border of the duodenum and the origin of the ileo-colic artery ; and pass outwards, or to the left side, towards their distribution on the small intestine.