By others, the spleen has been regard ed as a sponge, swelling with blood when the stomach is empty, and squeezed out by the pressure of the stomach when that organ is full. Thus, it is said, more blood will go into the stomachic arteries when the secretion of the gastric juice is going on, and the superfluous part in the inac tive state of the stomach will distend the spleen.
action of the email Intestines. The chyle, formed in the duodenum, passes through the tube of the small intestine, which is the organ for absorbing its nutritive parts.
The description of this fluid will be found under the article CHYLE. Its progress through the intestine is retarded by the numerous convolutions of the tube ; and the chyle, separated from the excrementitious part of the food, is brought into contact with the inhaling orifices of the lacteals, that commence, according to Lieberkiihn and other mi croscopical observers, by patulous orifices on the surface of the villi. The latter projections, so named from a comparison with the pill of velvet, are very nume rous on the circular projecting folds of the internal coat, called valvulx conni ventes. These latter not only render the progress of the chyle slower, but increase very greatly the absorbing surface, and penetrate, in consequence of the intesti nal contractions, into the midst of the chyle, in quest of its nutritive particles. A fluid is secreted from the intestine, analogous to that furnished by the sto mach,,Ahough an accurate examination • its nature and properties is one of the physiological desiderata (succus intesti nails). At the same time that the absorp tion of the chyle from the villous surface of the intestine is going on, it is moved gradually downwards, towards the large ila.eestine, by the peristaltic motion. This is an undulatory and gentle constriction, taking place in several parts of the tube at once, and producing therefore a singu lar appearance, compared to the crawling of worms, and hence termed vermicular. It moves the chyle repeatedly over the surface of the intestine ; and though it must urge that fluid partially upwards, yet its chief effect in the healthy state is exerted in the opposite direction. It is chiefly in disease that an antiperistaltic movement occurs, which conveys bile into the stomach, and even the whole contents of the small and large intes tine.
By the powers now explained, which propel the alimentary mass by their con tractions, and by the admixture of the various fluid menstrua which dilute and alter its properties, those memorable changes are effected, by which our food is said to be animalized or assimilated. In the duodenum and upper part of the jejunum it forms an equably mixed fluid, of the consistence of thick cream, grey ish, and rather acid. Lower down it se parates into two parts ; the excremen titious of a pale brown or yellow colour, and nauseating smell ; and the true chyle, separated from the former by the bile, and swimming on its surface.
action of the large intestine, The excre mentitious portion of the chyme, deprived almost entirely of every nutritive portion, enters the excum : its passage through the last part of the ileum being favoured by a copious secretion of mucous from the glands which abound in that part. The valvula coli, which is the boundary between the large and small intestines, is designed to prevent the contents of the former from regurgitating into the latter : and it performs this office in general; for nutritive clysters would otherwise enter the small intestines, and thus enable us to administer food enough for the support of life per anum : yet it occasionally fails in its office : hence vomiting of feces, and of tobacco clysters.
The large intestine may be regarded as the organ in which the residue of the chyme undergoes its last change ; the conversion into feces ; as a reservoir, protecting us from the disgusting neces sity of having that residue constantly flowing off; and as the instrument of its final expulsion from the body.
Some absorbents exist in this last part of the alimentary tube, and convey what nutritive parts still remain in the intesti nal contents ; but they are comparatively few, and hence the impossibility of nou rishing the body by way of clyster.
The conversion of the alimentary resi due into feces, is owing to some ac tive operation Of the intestine ; as these are very different from the mere putrid remains of animal or vegetable matter. And when this operation is deficient, portions of the food are seen in the eva cuations per anum, but slightly changed. The fecal matter is conveyed onwards by the peristaltic motion of the large intes tine : it become thicker and more con sistent in its progress, and is usually formed more or less decidedly by the cells of the colon. Its natural consist ence is just sufficient to retain these marks ; and its appearance throughout should be homogeneous. The colour de pends on the admixture of bile, and in the most natural state is of a yellow brown. Although the change which re duces the residue of our food to feces, is so far from being mere putrefaction, that the excrement is in fact less prone to pu trefy than other animal matter ; yet there is, as in putrefaction, a disengage ment of gaseous products, of sulphurated hydrogen. The colouring matter of the vegetables is often seen in the feces, as the green of spinach, and red of beet root ; and the fibrous indiges le parts of vegetables, as the skin of fruits, husks of grain, &c. The latter, indeed, is so little affected by the powers of digestion, that when the covering is entire, a grain may pass through the body, and still retain the power to ger minate.