C. MUSCULAR SYSTEM. 



THE muscles, commonly spoken of as flesh, may be divided 

 into two groups, according to their histological character, 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 smooth involuntary muscle-fibres preponderate in the 

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

 will ; the striated or voluntary muscles occur principally in the 

 body-walls and organs of locomotion, and are almost without 

 exception under the control of the will. The following general 

 statements refer exclusively to the latter kind of muscles. 



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 rule flat, while those of the extremities have usually an 

 elongated, cylindrical, or prismatic form. In some cases, however, 

 they assume the most various shapes ; for instance, there may be 

 more than one origin (bicipital, tricipital, or quadricipital forms), 

 the belly may be double (biventral or digastric form), or the muscle 

 may be saw-shaped, or have its fibres arranged in a single or double 

 series like a feather. 



All the muscles are surrounded by fibrous sheaths, or fasciae, 

 by means of which they are more or less firmly connected with 

 one another (intermuscular septa) and with the integument and 

 skeleton. 



A muscle may undergo very considerable modification both in form 

 and position by a change of origin and insertion, by division (intermediate 

 tendons dividing it into two or more parts), or by splitting into layers, and it 

 may thus give rise to one or more new independent muscles. If the action of a 

 muscle becomes unnecessary, it either disappears partly or entirely, or whut 

 remains of it contributes to the strengthening of a neighbouring muscle. 



In the embryo, the individual groups of muscles (e.g. the flexor sublimis 

 and profundus, and the masticatory muscles) are not at first separate from one 

 another, but consist of a uniform (mesoblastic) blastema, the splitting of which 

 takes place later by the ingrowth of fibrous septa. In this manner the indi- 

 vidual muscles are differentiated, and they gradually become more distinct by 



