804 THE CENTRAL NERVOUS SYSTEM 



the condition begins to pass off in from twenty minutes to half an hour, 

 after which a stimulus applied to the skin of the foot is followed by a 

 typical flexion movement at knee and hip, the so-called flexion reflex. 

 In the rabbit very little reflex response is elicitable for several hours 

 after the operation, but in a few days the reflexes return completely 

 below the level of the section. In the dog, on which a great deal of 

 work has been done, the involved regions of the body are profoundly 

 paralyzed. The skin is in a more or less unhealthy, unnatural condi- 

 tion, the surface cold, the hairs ruffled; and if care is not taken, the 

 slightest abrasion of the surface may result in a nasty ulceration. On 

 account of the paralysis of the centers of micturition and defecation, 

 there is also incontinence of urine and of feces. 



The Reflexes in the Spinal Animal 



With reasonable attention, however, the dog makes a wonderful re- 

 covery. After an interval of two weeks the hind limbs, although com- 

 pletely paralyzed so far as voluntary movement is concerned, begin to 

 show considerable signs of improvement. The first reflexes to return 

 are those concerned with the deeper structures, such as the vascular 

 reflexes, thus bringing the skin back to its normal temperature and 

 condition. The reflexes of micturition and defecation also soon return, 

 so that the animal no longer suffers from the continuous discharge of 

 urine and feces. About the same time the knee-jerk becomes elicitable. 

 This reflex is obtained by tapping the tendon which connects the patella 

 with the tibia, the response being a smart contraction of the extensor 

 muscles of the knee joint. The flexion reflex also begins to reappear. 

 This is elicited by applying a pinprick or other hurtful stimulus to 

 the skin of a lower extremity, and when fully developed consists in a 

 flexion of the knee and hip joints. The evident object of this move- 

 ment is that the stimulated parts may be removed from the source of 

 stimulation, and it is plain that all stimuli that produce the flexion 

 reflex are such as would cause in the intact animal a sensation of pain. 

 Such stimuli are thus classified as nocuous, and the reflex is styled a 

 nociceptive reflex. Accompanying flexion of the stimulated limb the op- 

 posite or contralateral limb usually undergoes a definite extension, 

 called the crossed extension reflex. The occurrence of this together with 

 the flexion of the stimulated limb is an important thing to remember 

 in testing the reflexes in man. Malingerers who attempt to make it ap- 

 pear that they have some lesion of the spinal cord may know that if 

 such lesion exists no movement of the leg occurs when the skin is 

 stimulated, but they are unlikely to know that under these conditions 

 the opposite leg also fails to show a simultaneous extension. 



