GENERAL BIOLOGY. 31 



sugar, and prepares the whole to pass further on its course : 

 when this has heen accomplished, the food is grasped and 

 squeezed and pushed along the tuhe, owing to the action of its 

 own muscular cells, into a sac (stomach), in which it is rolled 

 about and mixed with certain iiuids 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 con- 

 dition in which it can pass into the blood. Thus, all of the 

 food that is susceptible of changes of the Hnd 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 cov- 

 ering them which act as minute porters (villi). 



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 in a 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. 



