62 THE FROG CHAP. 



balanced by an increase in thickness, as when a stretched 

 piece of india-rubber is relaxed. 



The external influence by which a contraction is in- 

 duced is called a stimulus. As we have seen, a stimulus 

 may be produced by actual contact of some external 

 object (mechanical stimulus), or by chemical action 

 (chemical stimulus), or by heat (thermal stimulus), or by 

 an electrical current (electrical stimulus]. 



Relation of Muscle and Nerve. Evidently, however, 

 we have by no means got to the bottom of the matter. 

 In the living frog movements are always going on, and 

 all are due to the contraction of muscles, and yet no 

 stimuli of the kind enumerated are applied to any of 

 them. As the muscles retain the power of contraction 

 for some little time after the death of the animal, it is 

 easy to make such experiments as that described in the 

 next paragraph. 



Running longitudinally between the muscles on the 

 dorsal side of the thigh is a shining, white cord, the 

 sciatic nerve (Fig. 17, sc.nv), accompanied by an artery : 

 it gives off branches to the muscles and skin, and, 

 amongst others, one to the gastrocnemius. If, when 

 quite fresh, this nerve be carefully separated as it 

 traverses the thigh and pinched with the forceps, the 

 gastrocnemius will contract just as if the stimulus had 

 been applied to it directly, and the same will happen if 

 a chemical, thermal, or electrical stimulus be applied. 



Thus a stimulus applied to the nerve of a muscle has 

 the same effect as if applied to the muscle directly : it 

 gives rise to a nervous impulse, which, travelling along 

 the nerve, induces contraction of the muscle. 



Once more, however, external stimuli are not applied 

 to the frog's nerves during life, and it is obvious that we 

 must carry our inquiry a little further. The sciatic 

 nerve if traced upwards will be found to pass into the 



