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 smooth and those with transversely- 

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

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

 both in causing movements is dependent on the nervous system, a 

 nerve entering each muscle at a definite point. 



The smooth or involuntary muscle-fibres preponderate in the 

 viscera, derm, and vessels, and are not under the control of the 

 will ; the striated muscles occur chiefly in the body-walls and 

 organs of locomotion, and are almost without exception under the 

 control of the will (voluntary muscles). 1 The following general 

 statements refer exclusively to the latter kind of muscles, which 

 may, according to their mode of development, be arranged in the 

 following groups : 



'a. Muscles of the trunk, including the 

 coracohyoid of Fishes ( = sterno- 

 hyoid) and its derivatives in 

 higher Vertebrates : these repre- 



I. Parietal muscles de- 

 rived from the meso-^ 

 dermic somites. 



sent the oldest and most primitive 

 part of the muscular system. 



b. Muscles of the diaphragm. 



c. Muscles of the extremities. 



d. Eye-muscles. 



II. Visceral muscles, de- (Cranial muscles, with the exception 

 rived from the lateral -j of those included under a and d 

 plates of the mesoderm. \ above. 



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

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



1 Exceptions are seen in the muscles characteristic of the heart, and in 

 those of the alimentary canal in the Tench. More or less of the anterior part 

 of the digestive canal may contain striated fibres in various Vertebrates. 



