80 ANIMAL PHYSIOLOGY. 
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 com- 
position derived from cells on its inner surface, which trans- 
form 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 (vill). 
The impure blood is carried periodically to an extensive sur- 
face, usually much folded, and there exposed in the hair-like 
tubes referred to before, and thus parts with its excess of car- 
bon dioxide and takes up fresh oxygen. But all the functions 
described do not go on ina fixed and invariable manner, but 
are modified somewhat according to circumstances. The for- 
cing-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 car- 
ried from the brain and spinal cord along the nerves. All 
movements of the limbs and other parts are executed in obe- 
dience to its behests; and in order that these may be in accord- 
ance 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, 
