408 REFLEX ACTIONS. 



If turned over and placed on its back, it remains for an 

 indefinite period in that position, .without making any attempt 

 to regain its natural posture. While on its back, the heart 

 may be observed beating, but the respiratory movements will 

 be wholly absent. 



If thrown into a basin of water, it will sink to the bottom 

 like a Jump of lead (unless the lungs be too much distended 

 with air), without making any attempt whatever to swim. 



By irritating it in various ways, it may be made to execute 

 a variet}' of movements (see following observations), but can- 

 not, by any means, be made to leap or spring forward. 



Obs. II. With the point of a needle gentty stroke the 

 abdominal walls on one side. A slight twitching of the 

 muscles of the region so stroked will be witnessed. This is 

 one of the simplest forms of reflex action. Contraction takes 

 place in muscles on that side of the body only, the afferent 

 nerves of which are affected by the stimulus, and it will be 

 found that the afferent and efferent nerves concerned in the 

 action belong tolerably exactly to the same segment of the 

 spinal cord. 



On increasing the stimulus gradually by stroking more 

 forcibly, the twitchings will be seen to spread over a wider 

 and wider area, to invade the other side, and finally to pass 

 into the hinder and fore limbs. 



With a stimulus, sufficient^ slight, of an afferent nerve, a 

 definite small group of efferent fibres are alone affected by 

 reflex action. On increasing the intensity of the stimulus, the 

 effect spreads into a larger and larger number of efferent fibres. 



06s. III. Pass an S hook through the lower jaw, and thus 

 suspend the animal on a suitable upright, with the legs and 

 body hanging freely down. 



Or, take a piece of thin wood, about an inch broad and five 

 long ; place the frog, belly downwards, on it, in such a way 

 that the wood reaches no farther down than the lower part of 

 the abdomen, and secure the frog to it by two slight India- 

 rubber bands, one immediately below the fore limbs, and the 

 other a little above the thighs. If the wooden slip be now 

 fastened vertically in an upright, the hind limbs will hang 

 freely down, completely loose, while the body will be held 

 sufficiently firm. 



For most purposes the former simpler method is sufficient. 

 When it is desired to study the movements of the legs alone 

 with some accuracy, the latter method must be adopted. 



The legs hanging freely down, and the body being com- 

 pletely at rest, with a smooth pair of forceps gently pinch the 

 tip of one of the toes. The leg will immediately be drawn 

 sharply up, and after being kept in the flexed position for a 

 variable time, will be slowly dropped again. 



