94 INFLUENCE OF NERVOUS SYSTEM 



may occur even after section of both vagi, but effectual 

 vomiting is impossible. 



The Reflex of Defecation. As defaecation is possible 

 after severance of the spinal cord in the dorsal region, pro- 

 vided that the lumbar part of the cord is in a normal state, 

 it must be essentially a reflex process, governed by a spinal 

 centre. When accomplished under these conditions, it 

 consists of two acts, namely, peristaltic contraction of the 

 rectum and relaxation of the sphincter externus. The 

 mode of action of the latter, although it consists of striped 

 fibres, resembles, when not controlled by the will, that of 

 the cardia and pylorus : in its ordinary state it is con- 

 tracted, but it is excited to rhythmical relaxation by the 

 presence of fsecal matter in the neighbouring part of the 

 rectum. 



The Reflex of Micturition. The retention and discharge 

 of urine are also reflex acts, not necessarily dependent on 

 the will. The mechanism of micturition resembles, so far 

 as the bladder is concerned, that of defaecation, urine being 

 discharged after severance of the spinal cord, at long inter- 

 vals, by the simultaneous contraction of the muscular wall 

 of the bladder and relaxation of the sphincter, this pri- 

 mary act being accompanied by other auxiliary movements 

 accomplished by striped muscles acting under the direction 

 of the same centre. In the normal animal, the anal and 

 vesical sphincters are so far under the control of the will 

 that their relaxation can be inhibited by voluntary effort. 



Influence of the nervous system on the processes of Secre- 

 tion. The processes of secretion which have been hitherto 

 investigated, are those of the salivary glands, the pancreas, 

 the gastric glands, the liver and the kidney. 



The salivary glands are normally inactive, excepting 

 when excited by the presence of sapid substances in the 

 mouth. Hence the process is a reflex one. The centre 

 which governs it is in the medulla oblongata, and transmits 

 its influence to the submaxillary gland by the chorda 



