THE MEDULLA OBLONG ATA. 113 



Do the reflexes vary in strength with the strength of the 



stimuli ? 



A strong stimulus will excite a stronger reflex than a mild one. 

 A pin-prick will cause the foot to react more violently than tickling 

 the sole. In certain diseases and after certain poisons these actions 

 become very much exaggerated : tetanus and strychnine-poisoning 

 put the cord in such a condition that a very slight skin-irritation 

 may throw the entire body into a violent convulsive condition. 

 Again, other diseases or drugs decrease the irritability of the cord. 



What are special centres for reflex action? 



Certain actions are automatic that is, may take place without 

 intervention of the will in response to special stimuli and these 

 actions are dependent upon certain nerve-centres in the cord. 

 These centres have to do, noticeably, with sphincter action in the 

 pelvic region, and the centres themselves are located in the lumbar 

 region of the cord. To enumerate some of the automatic actions 

 of these centres is sufficient, as they are all dependent upon special 

 stimuli and special centres acting upon muscles peculiar to the 

 act. They are defecation, micturition, emission of semen, erection 

 of the penis, parturition. There are numerous other automatic 

 actions besides those of the pelvic sphincters, but the mode of 

 operation is well illustrated by these functions. 



What other automatic action occurs in the spinal nerve-centres ? 

 Vaso-motor centres and sweat-control centres are in the cord, 

 and centres which control the nutrition of the muscles, as well as 

 keep them tense in readiness to contract effectively ; that is, main- 

 tain the tone of the muscles. The nutrition of the entire body is 

 dependent upon the maintenance of this automatic control of the 

 vaso-motor function, as is shown by the disorders of the skin and 

 of the bones and joints which follow spinal injury and disease. 



What influence has the cord upon the functions controlled by the 



sympathetic system? 



Very great. The secretion of many of the body fluids and the 

 control of the organs is dependent for regulation upon the connec- 

 tion of. the sympathetic nerves with the spinal cord. 



THE MEDULLA OBLONG-ATA. 

 Describe, roughly, the medulla oblongata. 



It is a column of white and gray nerve-substance, and is an en- 



8 Phy. 



