C. MUSCULAR SYSTEM. 



The muscles, commonly spoken of as flesh, may be divided into 

 two groups, according to the histological character of their 

 elements, which consist of cells elongated to form contractile 

 fibres: namely, into those with smzoth and those with transversely- 

 striated fibres. The former are phylogenetically the older, and are 

 to he looked upon as the precursors of the latter. The action of 

 both in causing movements is dependent on the nervous system. 



The smooth or involuntary muscle-fibres preponderate in the 

 vascular system, viscera, and dermis, and are not under the control 

 of the will ; almost all the striated or voluntary muscles occur in 

 the body-walls and organs of locomotion, and are under the 

 control of the will.^ The following general statements refer 

 exchisively to the latter kind of miiscles, which may, according 

 to their mode of development, be arranged in the following 

 groups :— 



(a. Muscles of tlie trunk, including the 

 coracohyoid (sterno-hyoid) of 

 Fishes and its representatives in 

 I. Parietal muscles, de- higher Vertebrates : these repre- 



rived from the meso- / sent the oldest and most primitive 



blastic somites. 



part of the muscular system. 

 h. Muscles of the diaphragm. 



c. Muscles of the extremities. 



d. Eyc-musclcs. 



II. Visceral muscles, (^e- (Cranial muscles, \fith the exception of 

 rived from the lateral] those included under a and d 

 plates of the mesoblast. [ above. 



In its simplest form, an origin, a Iclhj, and an insertion, may be 

 distinguished in each muscle. The muscles of the trunk are as a 



^ Exceptions are seen in the muscles of the heart, and of the alimentary 

 canal in the Tench. More or less of the anterior and posterior parts of the 

 digestive canal may contain striated fibres in other animals. 



