130 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



tion causing them to contract in length and at the same time to 

 increase in diameter. 



There are two kinds of muscles which differ in origin, histological 

 appearance, physiological action and distribution. The smooth 

 muscles, the appearance of which has been described (p. 25), arise 

 from the me senchy me and are not under control o f Ihe wil l, but are 

 innervated by the sympathetic nervous system. Their action is 

 much slowe r than those of the other t3^e. They are found in the 

 skin, in the Avails of_blood-vessels and of the alimentary canal, and 

 in the u rogen ital system. Occasionally they occur as isolated fibres, 

 but frequently they form sheets or bands, sometimes of considerable 

 thickness. 



In the alimentary tract they are arranged in two layers in the 

 straight parts of the tube, an outer layer of fibres which run longitudi- 

 nally, and inside this a layer of circular muscles. In enlargements 

 of the tube this regularity is interrupted and the course of the fibres 

 is more irregular. The circular musc les, by their contraction, lessen 

 the diameter o f the canal, at the same time causing it to elongate, 

 while the longitudinal fibres shorten Jt and cause it to increase in 

 diameter. By the action of these two layers, peristalsis is produced, 

 the food being moved forward and backward, thus aiding in its diges- 

 tion and absorption. In the b^od-vessels there are only ci rcula r 

 fibres, the enlargement of the lumen being caused by the internal 

 blood pressure. 



The stripedniusclesare derived from the walls of thcLCoelom and 

 hence are of mesothelial _o riffln. Excepting those of the heart (to 

 be mentioned below) and some of those at the anterior end of the 

 alimentary canal, they are undfiUCfl ntrol of the wj ]l and are supplied 

 by the motor nerves of the brain and spinal cord. They are also able 

 to^cojitract mor e rapidly than the smooth muscles. The striped 

 muscles make up the great mass of the musculature — the ' flesh ' — 

 of the body. They occur in the body walls, organs of locomotion, 

 the head, diaphragm and the anterior part of the digestive canal. 



The voluntary muscles are derived in part from the somites 

 (myotomes), in part from the lateral plates, the latter furnishing the 

 visceral muscles, including those of the head (except the eye muscles 

 and the sternohyoid and its derivatives in the higher vertebrates) and 

 those of the heart. The heart muscles, the development of which is 

 traced in the account of the circulatory system, differ from the other 

 striped muscles in the uninucleate condition of their short and 



