REFLEX ACTION 485 



excited by the reflex action of the cord, is not regulated 

 alone by the number of the afferent nerves which are 

 stimulated by the irritation which gives rise to the reflex 

 action. Nor does a simple excitation of the afferent nerve 

 by any means necessarily impl}- a corresponding simplicity 

 in the arrangement and succession of the reflected motor 

 impulses. Tickling the sole of the foot is a very simple 

 excitation of the afferent fibres of its nerves ; but in order 

 to produce the muscular actions by which the legs are 

 drawn up, a great multitude of efferent fibres must act in 

 regulated combination. In fact, in a multitude of cases a 

 reflex action is to be regarded rather as the result of a 

 dormant activity of the spinal cord awakened by the 

 arrival of the affei'ent impulse, as a sort of orderly explosion 

 fired off by the afferent impulse, than as a mere rebound 

 of the afferent impulse into the first efferent channels 

 open to it. 



The various characters of these reflex actions may be 

 very conveniently studied in the frog. If a frog be deca- 

 pitated, or, better still, if the spinal cord be divided close to 

 the head, and the brain be destroyed by passing a blunt 

 wire into the cavity of the skull, the animal is thus de- 

 prived (by an operation which, being almost instantaneous, 

 can give rise to very little pain) of all consciousness and 

 volition, and yet the spinal cord is left intact. At first the 

 animal is quite flaccid and apparently dead, no movement 

 of any part of the body (except the beating of the heart) 

 being visible. This condition, however, being the result 

 merely of the so called sl)ock of the operation, very 

 soon passes off, and then the following facts may be 

 observed. 



So long as the animal is untouched, so long as no 

 stimulus is brought to bear upon it. no movement of any 

 kind takes place : volition is wholly absent. 



If, however, one of the toes be gently pinched, the leg is 

 immediately drawn up close to the body. 



If the skin between the thighs around the anus be 



