210 COMPARATIVE PHYSIOLOGY. 



To return to reflex action, it is found that the muscular re- 

 sponse to a peripheral irritation varies with the point stimu- 

 lated, the intensity of the stimulus, etc., hut is, above all, deter- 

 mined by the central cells. 



Nerve influence may be considered as following lines of 

 least resistance, and there is much evidence to show that an im- 

 pulse having once taken a certain path, it is easier for it to pass 

 in this direction a second time, so that we have the foundation 

 of the laws of habit and a host of interesting phenomena in 

 this simple principle. 



It is found that, in a frog deprived of its brain and sus- 

 pended by the under jaw, there is no movement unless some 

 stimulus be applied ; but if this be done under suitable condi- 

 tions, instructive results follow, which we now proceed to indi- 

 cate briefly. The experiments are of a simple character, which 

 any student may carry out for himself. 



Experimental. — Preparing a frog by cutting off the whole 

 of the upper jaw and brain-case after momentary anaesthesia, 

 suspend the animal by the lower jaw and wait till it is perfectly 

 quiet. Add to water in a beaker sulphuric acid till it tastes 

 distinctly but not strongly sour, to be used as a stimulus. 1. 

 Apply a small piece of bibulous paper, moistened with the acid, 

 to the inner part of the thigh of the animal. The leg will be 

 drawn up and the paper probably removed. Remove the paper 

 and cleanse the spot. 2. Apply a similar piece of paper to the 

 middle of the abdomen ; one or both legs will probably be 

 drawn up, and wipe off the offending body. 3. Let the foot of 

 the frog hang in the liquid ; after a few moments it will be 

 withdrawn. 4. Repeat, holding the leg ; probably the other 

 leg will be drawn up. 5. Apply stronger acid to the inside of 

 the right thigh ; the whole frog may be convulsed, or the left 

 leg may be put in action after the right. Even if the stimulat- 

 ing paper be applied near the anus, it will be removed by the 

 hind-legs. 6. Beneath the skin Df the back (posterior lymph- 

 sac) inject a few drops of liquor strychnise of the pharama- 

 copoeia; after a few minutes apply the same sort of stimulus to 

 the thigh as before. The effects follow more quickly and are 

 much more marked — the animal, it may be, passing into a gen- 

 eral tetanic spasm. 



These experiments may be varied, but suffice to establish the 

 following conclusions : 1. The stimulus is not . immediately 

 effective, but requires to act for a certain variable period, de- 



