84 A COURSE ON ZOOLOGY. 



as those of the limbs, of the face, etc. These are volun- 

 tary muscles. 



2. Those whose action cannot be controlled by the 

 will, such as those of the heart, of the muscular coats of 

 the intestines and stomach, etc. These are involuntary 

 muscles. 



The differences in individual vigor are essentially de- 

 pendent on the greater or less development of the mus- 

 cular system. This development may be natural, but it 

 is frequently the result of suitable exercise and appro- 

 priate diet and habit of life. So physicians prescribe for 

 young persons of deficient development different exer- 

 cises for the body, such as fencing, boxing, swimming, 

 etc. The ancients formed their athletes in the same 

 manner, and even our horses are not permitted to com- 

 pete at the races without previous training-. 



The contraction of a set of muscles cannot be pro- 

 longed beyond a certain time, always quite limited. A 

 continuation of the state of contraction brings about 

 fatigue, and the necessity for a cessation of the muscular 

 tension. 



As for the involuntary muscles, their intermittent 

 action continues throughout the course of life without 

 producing any direct sensation of fatigue ; and yet, to give 

 but a single example of the work they accomplish, the 

 heart beats about one hundred thousand times a day. 



The Voice and Vocal Apparatus. The sounds which 

 constitute the human voice are produced by the passage 

 of the air expelled from the lungs through a very simple 

 apparatus called the larynx. 



This apparatus is in the upper part of the trachea, and 

 is a tube formed by the union of a number of elastic, 

 cartilaginous rings, separated from the pharynx by the 



