294 FIRST YEAR COURSE IN GENERAL SCIENCE 



same way that its parents, its grandparents, and all its 

 ancestors did. They lived to adult life because they suc- 

 ceeded in their methods. The kitten has inherited the 

 habits of the race. 



342. The Structure of Muscles. In the simplest ani- 

 mals, the entire body is concerned with the process of moving. 



In the higher animals, 

 movements are brought 

 about by organs called 

 muscles. Muscles are 

 voluntary or involun- 

 tary, according to their 

 control by the animal. 

 The muscles which may 

 be controlled by the will 

 are voluntary muscles. 

 They consist of masses 

 of elongated cells lying 

 parallel to one another 

 and arranged in small bundles, which also lie parallel to one 

 another. These bundles may easily be seen in boiled meat, 

 where they tend to separate from one another as strings or 

 fibers. Each end of a muscle is firmly attached by a tough 

 gristly cord, called a tendon, to some part of the body gen- 

 erally to a bone. 



The involuntary muscles are not fibrous in structure and 

 are not under the control of the animal. The muscles of the 

 digestive organs show the structure of involuntary muscles. 

 343. The Work of Muscles. In the division of labor 

 which exists among the cells of animals, the muscle cells have 

 been given the work of contraction, or shortening and thick- 

 ening, but they act only under the direction of the nervous 

 system. In most muscles, all the cells contract at the same 

 instant, when so directed by some part of the nervous sys- 

 tem. The muscle .itself, therefore, shortens and grows 



FIG. 155. A BUNDLE OF MUSCLE 

 FIBERS, (much magnified) 



This picture represents a "string," 

 such as one sees in boiled lean meat. Each 

 string is composed of hair-like threads, rep- 

 resented by one of the small parts of this 

 bundle. 



