252 PHYSIOLOGY FOR DENTAL STUDENTS. 



animal, say a dog, in so far as voluntary movement is con- 

 cerned, is entirely paralyzed in all portions of the body below 

 the level of the section of the cord. It cannot voluntarily move 

 the affected parts, it cannot walk, it feels no pain or any other 

 sensation below the lesion, and yet when appropriately stimu- 

 lated, the paralyzed limbs may reflexly undergo various, often 

 very complicated movements. 



The Essential Characteristics of Reflex Action. As studied 

 on a perfectly recovered spinal dog these are as follows : 



1. For a certain interval after applying the stimulus then- 

 is no response, the duration of this "latent period" depending 

 partly on the nature of the reflex (short in the protective re- 

 flexes, long in the scratch reflex) and partly on the strength of 

 the stimulus. 



2. The response may persist for some time after the stimulus 

 is removed (after response). 



3. The degree of the response is roughly proportional to the 

 strength of the stimulus, except in certain of the protective re- 

 flexes, such as the conjunctival, which consists in the closing of 

 the eyelids when anything touches the eye. 



4. The response is often rhythmical in character, even though 

 the stimulus be continuously applied. This is well seen in the 

 scratch reflex. 



5. There are certain ways, apart from an alteration in the 

 stimulus, by which we may cause a reflex movement to become 

 increased or decreased. Thus, taking the flexion reflex as an 

 example, the flexion may be diminished: (1) by stimulating 

 some other reflex movement which involves the same muscles, 

 but which is antagonistic to flexion, e.g., by stimulating the 

 opposite limb and causing the so-called crossed extension reflex ; 

 (2) by causing strong afferent impulses to pass through other 

 levels of the spinal cord, e. g., pinching the tail. A similar 

 "interference" is well illustrated in the case of man by stimulat- 

 ing the fifth nerve by firm pressure on the upper lip at a time 

 when there is an inclination to sneeze. The snee/ing, which is 

 a reflex due to irritation of the mucosa of the nose, can usually 

 be prevented. Expressing this phenomenon of reflex iiiterfer- 



