While the dissolution of the food, pro. duced by the solvent action of the gastric juice, is going on, the two orifices of the stomach remain accurately closed. No gas ascends through the cesophagus, ex cept when the digestive process is imper fect. Soon the muscular fibres of the or gan begin to act : the circular ones, con tracting at first in a vague and oscillatory manner, soon act more uniformly from above downwards, and from right to left ; that is, from the cesophagus to the pylo rus ; while the longitudinal part approxi mates the two openings. The pylorus seems to possess a peculiar and exquisite sensibility, by which it distinguishes whe ther the substances brought in contact with it have been sufficiently acted on by the gastric juice ; if that is the case, it re leases and allows them to pass, while it remains closely contracted against those which are not thoroughly digested.
The time occupied by the digestive pro cess must be expected to vary according to the constitution, age, and health of the individual, and the nature of the aliment ; but it may be stated, in general, at four hours.
The action of the stomach is sometimes inverted, and the contractions, which in that case are forcible, rapid, and convul. sive, cause vomiting. The exertions of the respiratory muscles are, however, ne cessary to the production of this effect.
Although the stomach belongs to those organs whose action is independent of the will, and goes on, therefore, without the attraction of the individual, yet it is so far influenced by the brain, that the section of its nerves entirely obstructs digestion. It sympathises most remarkably both with the constitution at large, and with parti cular organs.
The chyme, or semifluid substance, in. to which the action of the stomach reduces the food, is propelled by the muscular power of that bag into the duodenum, where it undergoes new changes, particu larly from the admixture of other animal fluids, the bile and pancreatic juice : and this process is termed chylification.
The secretion of the pancreas seems to bear a considerable analogy, both in its nature and office, to the saliva and gas tric juice. It is very difficult to procure it in a pure and unmixed state from a healthy animal, but the facts which we hitherto possess, spew, that its properties are very like those of saliva. It is poured into the duodenum by the contractile pow er of the pancreatic duct : and its secre tion, like that of the salivary glands, is said to be promoted by the pressure of the stomach in its filled state; and by the stimulus of the chyme and bile on the ori fice of its duct.
The bile is the fluid furnished by the liver, the largest viscus of the human body, whose importance in the animal economy is evinced by its large apparatus and complicated distribution of blood vessels, as well as by its constantly exist ing in all animals which have a heart.
The source of the bile has been ques tioned, whether it be secreted from the hepatic artery, or vena portarum The analogy of other secretions, which are all made from arteries, favours the former opinion ; but the blood of the vena porta rum seems to be the fittest, from its na ture, for the secretion of bile. The hepa tic artery probably serves to nourish the liver, as the lungs are nourished by the bronchial artery. The distribution of the vena portarum is like that of an artery. If we suppose the hepatic artery to furnish the bile, the excretory duct would be dis proportionately large.
The bile, when secreted, passes by a slow but constant stream through the he patic duct ; but a greater or less pro. portion is conveyed by the cystic duct into the gall-bladder, in which it is accumu lated, as in a reservoir, and undergoes certain changes of properties, which make it cystic bile. The biliary fluid probably flows directly into the intestine, while the assimilation of the food is going on ; and, when the empty state of the duodenum causes the end of the ductus communis choledochus to be compressed by its mus cular coat, it regurgitates into the gall. bladder.
There is no direct communication be. tween the liver and gall-bladder, and con. sequently no other way for the bile to en ter that bag, except by the cystic duct. The hepatic bile is a thin fluid, of a clear orange colour, and slightly bitter ; the cystic is viscid and tenacious, intensely bitter, and very dark coloured. The bile, thus changed by its residence in the gall bladder, is expelled by the contractile powers of the gall-bladder and cystic duct into the ductus communis, and thence passes to the duodenum. This bladder absorbs the watery parts of the hepatic bile, and adds a mucous secretioh to it. The chemical analysis, and theNes of the bile, are considered under that article. Function of the Spleen. The situation and attachments of this organ lead us to suppose, that its uses are in some way connected with the functions of the sto mach ; yet there is nothing more than conjecture to be offered oft this subject. The removal of the part has been per formed in dogs without any material in jury to the animal. Its size differs, ac cording the quantity of blood contained in its cells. Physiologists have stated, that its blood possesses peculiar properties ; that it is more fluid, does not coagulate, nor separate readily into serum and eras samentum ; is more livid, and possesses a greater quantity of carbon. As this blood goes to the liver, and as the part possesses no excretory duct, it has been argued, that its function is subservient to that of the liver, in imparting to the blood those properties which fit it for the secre tion of bile.