REFLEX ACTION. 251 



in the paralyzed region, the first to do so being the protective 

 reflexes, of which the flexion reflex is the type. 



The flexion reflex is elicited by any stimulus which would cause 

 pain in an animal capable of feeling. Such stimuli are called 

 nocuous and the reflex response is always of such a nature 

 usually flexion as to cause the injured part to be removed 

 from further damage. The return of the flexion reflex is soon 

 followed by that of the knee jerk, which is elicited by tapping 

 the.patellar tendon after putting it on the stretch by passively 

 bending the knee joint. Somewhat later in many animals (e.g., 

 dog) the scratch reflex appears, so-called because it consists of 

 a scratching movement of the hind leg in response to mechanical 

 irritation* of the flank of the animal! It is a reflex of very great 

 interest because it illustrates to what a remarkable degree the 

 spinal cord, unaided by the brain, is capable of bringing about 

 complicated and purposeful co-ordinated movement. Later still, 

 in the lower animals, practically all the reflex movements which 

 a normal animal exhibits may reappear. 



When the cord becomes severed in man, as by spinal fracture, 

 spinal shock is extremely profound, and in order to keep the 

 patient alive great care must be taken, on account of the incon- 

 tinence of urine, to prevent infection of the bladder and kidneys 

 and to protect the skin from ulceration (bed sores). Even in 

 such cases, however, many of the reflexes recover in the para- 

 lyzed regions, but the recovery is slow and the limbs invariably 

 atrophy. It is particularly important to note that the time of re- 

 appearance of the reflexes bears a relationship to the degree of 

 development of the cerebral hemispheres, thus rendering it evi- 

 dent that spinal shock is due to a break in the nerve paths which 

 lead to and from the brain. The higher the animal, the more 

 frequently do all reflex acts involve a cerebral path instead of 

 taking the short cuts available through the collaterals (see p. 

 243). From usage, as it were, the cerebral paths become so well 

 developed that when they are suddenly severed, the reflex action 

 becomes impossible until the entering afferent impulse has 

 learned to use the hitherto unused short cuts available through 

 collaterals. When completely recovered from spinal shock, an 



