To secure full and natural action in the intestinal canal, several principal conditions are thus necessary, failure of any of which may impair their activity. The first condition is well-digested chyme and chyle; the second, a due quantity and quality of mucous and vascular secretions from the villous coat; the third, full contractile power in the muscular fibres of the intestine, and free action of the abdominal and respiratory muscles; and the last a due nervous sensibility to receive impressions and communicate the necessary stimulus. And hence, when the bowels act imperfectly, it is of importance to ascertain to what cause the inability is to be ascribed, that an appropriate treatment may be devised. * * * * * Such are the general structure and uses of the intestinal canal; but there are modifications in its individual portions on which it may be right to offer a few additional remarks. We shall begin with the _duodenum_. The _duodenum_ (from _duodeni_, twelve, being considered equal in length to twelve finger-breadths) commences at the pyloric orifice of the stomach, from which it crosses over under the lower surface of the liver, towards the right side; it then descends in front of the right kidney, and there forming a second curve, it proceeds again towards the left, and a little beyond the spinal column terminates in the jejunum. It thus describes a course like the letter C, and has its convexity turned towards the right, while the _pancreas_ or sweetbread (PP) lies in the space enclosed by its concavity. To enable the reader to form some notion of the relative position of these parts, I have introduced a wood-cut on the next page, shewing the situations and appearance of the different organs after the intestines, as in the figure on page 155, have been removed from the body. The letters LLLL point out the inferior surface of the liver, a little raised from its natural position to shew the gall-bladder G and the pancreas PP, round the right end of which the duodenum is curved. S indicates the spleen, with a vacant space over it, in which the stomach lies. The kidneys, KK, lie one on each side of the spine; and the two pipes UU are the ureters which convey their secreted fluid to the bladder B. The letters AA indicate the great artery (the _aorta_), through which the nutritive blood descends to supply the bowels and lower parts of the body; and VV mark the corresponding vein (the _cava_), by which the dark blood returns from the extremities towards the heart. R is the beginning of the _rectum_ or straight gut seen at YY in the cut on page 155. [Illustration]