FUNCTIONS OF LOCOMOTION. 83 



Muscular tissue is composed of parallel fibres arranged 

 in bundles. The fibres are of microscopic dimensions, 

 and the separate bundles are all enveloped by a fibrous 

 covering, called aponeurosis ; an envelope of the same 

 nature surrounds the whole muscle and isolates it from 

 the surrounding parts. 



Ordinarily the muscles have elongated forms, like 

 spindles ; the middle portion, composed of muscular tis- 

 sue, strictly speaking, is red and fleshy, and constitutes 

 the meat of edible animals. The two extremities of the 

 muscle are formed of a very strong, white, pearly sub- 

 stance, and this is a direct continuation of the periosteum 

 or fibrous envelope covering the bones. The end of the 

 muscle connected with the bone which the muscle moves 

 is usually elongated like a cord, is called the tendon, and 

 is the origin of the muscle; that end which is attached 

 to the fixed bone is usually wide and flattened, and forms 

 what is called the aponeurosis of insertion. 



The life of the muscles is very active, and they there- 

 fore receive many blood-vessels. They are also provided 

 with many nervous filaments, but most of these are from 

 the motor system, and the sensibility is not highly de- 

 veloped. 



We cannot enter into details concerning the muscles. 

 These organs, which number more than four hundred, 

 are symmetrically arranged, and usually by pairs. They 

 form several superposed layers. Figures 43 and 44 show 

 the superficial muscles of the trunk, that is, those 

 which in this region are placed immediately under the 

 skin. 



So far as their primary action is concerned, the muscles 

 are divided into two classes : 



1. Those whose action is controlled by the will, such 



