214 EXPERIMENTAL PHYSIOLOGY 



CHAPTEE XXI 



REFLEX ACTION AS STUDIED UPON THE SPINAL CORD OF THE FROG 



Experiment 1. — Destroy the brain of a frog, leaving the spinal cord intact. 

 This should be done with a blunt instrument, so that the animal loses as 

 little blood as possible. If the animal be tested immediately by pinching one 

 of the toes it may or may not respond by a movement of the leg. If it be 

 kept for half an hour or a little longer it will very readily respond. The 

 absence of response just after the destruction of the brain is due to the shock 

 of the operation, but this rapidly passes off hi the case of the frog. 



After this period of rest note the condition of the frog. It lies on 

 the table without any attempt at spontaneous movement. Its head 

 and body lie in contact with the table, whilst in an intact frog they 

 are always inclined to the surface on which it rests. The legs are 

 usually drawn up and the fore limbs may be extended at right angles 

 to the axis of the body or may lie folded over the sternum. The eyes 

 are closed and no respiratory movements are attempted. The general 

 attitude of the animal should be contrasted by comparison with that 

 of an intact frog. 



Having determined these points its behaviour when its position is 

 altered should be investigated. If a leg be stretched out it is usually 

 drawn up again to its original position as soon as the fingers are 

 withdrawn. Place it on its back ; it will lie at rest practically in the 

 position in which it is placed. It makes no attempt to turn over into 

 its previous position, whereas an intact frog immediately turns over 

 as soon as it is allowed to. Suspend the frog by passing a bent pin 

 through the lower jaw. The pin does not act as a stimulus, and so 

 cause reflex movements, because the centres of the sensory nerves of 

 this part have been destroyed in pithing the brain. The frog may thus 

 be supported in any convenient manner and the reflex movements in 

 response to various forms of stimuli studied. 



1. Mechanical. 



(a) Pinch any one of the toes of the right foot ; the right leg is drawn 

 up. If the toe be held you will feel a puil on the fingers, tending to 

 lift the leg. This pull is not continuous, but varies in strength. Pinch 

 a toe of the left foot ; the left leg is drawn up. Pinch the skin of the 



