THE TISSUES 2J 



self often kills the bacteria ; while on the other hand the 

 white cells are not always able to destroy the bacteria, 

 even by devouring them. There are reasons for think- 

 ing that the white cells may secrete a substance poison- 

 ous to bacteria, and thus destroy them without con- 

 suming them. 



5. Muscle is the tissue concerned with movement, Muscle 

 which may be either that of a part of the body, as the 

 heart, or that of the whole organism. There is, of 

 course, a great deal of movement going on, as, for ex- 

 ample, that of absorption, which is not controlled by 

 muscle ; but the gross and obvious movements are nearly 

 all muscular. Muscle cells are elongated, like very 

 slender worms attached together in bundles. Their 

 function is, of course, to contract,, which they do in 

 response to a stimulus. This is not in itself a special 

 function of these cells ; the primitive one-celled amiba 

 also contracts under suitable conditions. The muscle 

 cell, however, has the same sort of relation to an amiba 

 that an express train has to a person walking along the 

 road. The walker does many things which the train 

 does not, but the train is extraordinarily specialized for 

 going, and for going in a particular way along a par- 

 ticular track, under the control of an engineer. In the 

 case of the muscle, the engineer is the nerve. 



Those muscle cells which are under voluntary control Voluntary 

 are striated, showing fine cross-lines in the manner of a 

 file. This is equally true in vertebrate and invertebrate 

 animals. Unstriated muscle is not under the control of 

 the will ; such, for example, is the muscle which causes 

 the movements of the intestine. The distinction here 

 made is not absolute, however, for the heart muscle is 

 striated. The fibers of the heart are in fact somewhat 

 intermediate in structure between the two great classes 



