MUSCULAR SYSTEM. 125 



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. 20), arise 

 from the mesenchyme and are not under control of the will, but are 

 innervated by the sympathetic nervous system. Their action is much 

 slower than that of the other type. They are found in the skin, in the 

 walls of blood-vessels and of the alimentary canal, and in the urogenital 

 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 longitudinally, 

 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 muscles, by their contraction, lessen the diam- 

 eter of the canal, at the same time causing it to elongate, while the 

 longitudinal fibres shorten it and cause it to increase in diameter. In 

 the blood-vessels there are only circular fibres, the enlargement of the 

 lumen being caused by the internal blood pressure. 



The striped muscles are derived from the walls of the coelom and 

 hence are of mesothelial origin. Excepting those of the heart (to be 

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

 1 canal, they are under control of the will and are supplied by the motor 

 nerves of the brain and spinal cord. They are also able to contract 

 more 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 (myo- 

 tomes), in part from the lateral plates, the latter furnishing the vis- 

 ceral 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 usually branched 

 cells, while, physiologically, they are involuntary in character. 



THE PARIETAL MUSCLES. 



After the myotomes are cut off from the rest of the ccelomic walls 

 (p. 14) each consists of a closed sac, containing a part of the ccelom 



