62 THE FROG CHAP. 



decrease in length is balanced by an increase in thickness, 

 as when a stretched piece of india-rubber is relaxed. 



The external influence by which a contraction is induced 

 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 are all 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 experi- 

 ments 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 a vein : it gives off 

 branches to the muscles and skin, and, amongst others, one 

 to the gastrocnemius. If 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 trunk (Fig. 51, Sci), 



