3l8 PRACTICAL PHySIOLOGT. [LXVIIL 



PHYSIOLOGY OF THE CENTRAL NERVOUS 

 SYSTEM. 



LESSON LXYIIL 



REFLEX ACTION— ACTION OF POISONS- 

 KNEE-JERK. 



1. Reflex Action. — Destroy the brain of a frog down as far aa 



the medulla oblongata, which should be done without loss of blood. 

 Place under a bell-jar a normal frog for comparison. Immediately 

 the frog is pithed, on pmching one of its toes, very probably the 

 leg will not be drawn up. After half an hour or more (by this 

 time it has recovered from the shock of the operation), observe — 



(a.) Its attitude : the head of the pithed frog lies on the plate 

 on which it is placed, while in the intact frog the head is erect, 

 the body and head forming an acute angle with the surface on 

 which the frog rests. 



(h.) Its eyes are closed, while those of the intact frog are open. 

 The fore-limbs are either .flexed and drawn under the chest, or 

 spread out, so that the body is no longer supported on the nearly 

 vertical fore-limbs, as in the intact frog, but lies flat upon the 

 surface of support. The legs are pulled up towards the body. 



(c.) The absence of respiratory movements in the nostrils and 

 throat. It makes no spontaneous movements, if left entirely to 

 itself. 



(d.) Turn it on its back : it lies in any position it is placed. Do 

 this witli a normal frog ; the latter regains its equilibrium at once. 

 Extend one of the legs ; it will be drawn up again towards the 

 body. Pinch the flank with a pair of forceps ; the leg of the same 

 side is rapidly extended, then drawn up towards the spot stimulated. 

 Pinch sharply the skin round the anus with forceps. Immediately 

 both legs are pushed out and pulled up towards the body, as if to 

 dislodge the ofi'ending body. 



2. Bend a long (6 cm.) straight pin into the form of a hook, 

 and push it through the tips of both jaws, and by means of the 

 hook hang up the frog vertically on a suitable support. At first 



