680 PHYSIOLOGY. 



out a contraction of one set of muscles will inhibit the antagonistic 

 muscles. 



Sherrington has shown that the reflex arc in the knee-jerk is 

 due to nerve-fibers passing to and from the quadriceps extensor by 

 the anterior crural nerve, and to those passing to and from the ham- 

 string muscles by the sciatic. 



The tendon reflexes are not true reflexes, but are due to a direct 

 stimulant action on the muscle itself. But a reflex arc is necessary 

 to keep the muscles in a state of tonus that the tendon reflexes may 

 take place. 



Centers in the Spinal Cord. The spinal cord presides over the 

 movements of the anus, the bladder, and the genital apparatus by 

 means of three centers located one above the other. 



The ano-spinal center is found in the dog near the fifth lumbar 

 vertebra. From this center emanate fibers which, with the sacral 

 nerves, go to animate the sphincter of the anus. Irritation of this 

 center, especially by disease, brings on spasm of the sphincter, with 

 difficulty in passing faeces. Destruction of the center causes paralysis 

 of the sphincter and incontinence of faeces. 



In paraplegics (those affected with paralysis of the lower limbs 

 from cord lesion), spinal incontinence or the involuntary passage of 

 the faeces may be observed. Or there is a protracted and invincible 

 constipation. The former condition depends upon the destruction 

 of the spinal center, while the latter comes from paresis of the intes- 

 tine in the region of the colon and rectum. 



The vesico-spinal center in dogs is found between the third and 

 fifth lumbar vertebra. When it is stimulated or the nerves which 

 take their departure from it, there are energetic and painful contrac- 

 tions of the body and neck of the bladder. 



In apoplectics there is often, first, ischuria (retention of urine), 

 which seldom comes from irritative or nervous spasm of the sphincter, 

 but more frequently from paralysis limited to the detrusor nerves 

 only. Afterward there is enuresis (incontinence of urine), from 

 paralysis also of the nerves of the sphincter. 



The genito-spinal center is to be found in the spinal cord at the 

 level of the fourth lumbar vertebra. If excited by stimuli it pro- 

 duces contractions of the lower part of the rectum, bladder, and, if 

 the animal be a female, the uterus. In addition, if the spinal cord 

 be cut between the dorsal and lumbar parts, tickling of the mucous 

 membrane of the glans penis of the dog determines by reflex action 

 an erection. Erection is no longer obtained if the lumbar cord be 



