iv MUSCULAR SYSTEM 63 



trunk (Fig. 51, sci), and finally to join the spinal cord, 

 which, as we have seen, is in connection with the brain. 

 In the living frog nervous impulses originate in the 

 brain, without the direct intervention of an external 

 stimulus, and are conducted along the cord and nerves 

 to the muscles. But further consideration of this sub- 

 ject must be deferred until we have made a special study 

 of the nervous system. 



The Muscular System in General. All over the body 

 the muscles, though varying greatly in form some 

 being elongated and band-like (Fig. 16, sar), others 

 spindle-shaped (gastr), others in the form of broad, flat 

 sheets (my. hy, obi. ext) have the same general relation 

 to the skeleton as in the case of the gastrocnemius. 

 Each muscle arises or has its origin in a relatively fixed 

 part, and is inserted into a relatively movable part. 

 As each muscle contracts in one direction only, it follows 

 that the more complex the movements any part is 

 capable of performing, the more numerous must be its 

 muscles. For instance, the femur, which, as we have 

 seen, is capable of universal movement, has no fewer 

 than nine muscles, arising from various parts of the hip- 

 girdle, inserted into it. Even the minute phalanges of 

 the fingers and toes all have their little slips of muscle 

 by which the various movements of grasping and 

 relaxing, approximating and separating the digits, are 

 effected. 



There are certain terms applied to muscles which it is 

 useful to know. A muscle which raises a part, e.g., the 

 lower jaw, is called a levator, one which lowers a part a 

 depressor. A muscle which serves to straighten one part 

 upon another, e.g., to bring the shank into line with the 

 thigh, is an extensor, one which bends one part on another is 

 a flexor. A muscle which draws, e.g., a limb towards the 

 trunk is an adductor, one which draws it away an abductor, 

 one which rotates one part upon another (e.g., the femur on 

 the pelvis), a rotator 



