Page:A Text-book of Animal Physiology.djvu/60

30 mixed with a fluid which changes the starchy part of it into sugar, and prepares the whole to pass further on its course: when this has been accomplished, the food is grasped and squeezed and pushed along the tube, owing to the action of its own muscular cells, into a sac (stomach), in which it is rolled about and mixed with certain fluids of peculiar chemical composition derived from cells on its Inner surface, which transform the proteid part of the food into a form susceptible of ready use (absorption). When this saccular organ has done its share of the work, the food is moved on by the action of the muscles of its walls into a very long portion of the tract in which, in addition to processes carried on in the mouth and stomach, there are others which transform the food into a condition in which it can pass into the blood. Thus, all of the food that is susceptible of changes of the kind described is acted upon somewhere in the long tract devoted to this task. But there is usually a remnant of indigestible material which is finally evacuated. How is the prepared material conveyed into the blood? In part, directly through the walls of the minutest blood-vessels distributed throughout the length of this tube; and in part through special vessels with appropriate cells covering them which act as minute porters (villi).

The impure blood is carried periodically to an extensive surface, usually much folded, and there exposed in the hair-like tubes referred to before, and thus parts with its excess of carbon dioxide and takes up fresh oxygen. But all the functions described do not go on in a fixed and invariable manner, but are modified somewhat according to circumstances. The forcing-pump of the circulatory system does not always beat equally fast; the smaller blood-vessels are not always of the same size, but admit more or less blood to an organ according to its needs. This is all accomplished in obedience to the commands carried from the brain and spinal cord along the nerves. All movements of the limbs and other parts are executed in obedience to its behests; and in order that these may be in accordance with the best interests of each particular organ and the whole animal, the nervous centers, which may be compared to the chief officers of, say, a telegraph or railway system, are in constant receipt of information by messages carried onward along the nerves. The command issuing is always related to the information arriving.

All those parts commonly known as sense-organs—the eye,