204 TEXT-BOOK OF PHYSIOLOGY. 



coming from the cord are preganglionic, those coming from the ganglia, 

 postganglionic. 



The central mechanism that excites and coordinates the activities of the 

 rectal muscles is situated in the lumbo-sacral region of the spinal cord and 

 is designated the recto-anal center. 



The afferent nerve fibers that excite the central mechanism to activity, 

 are contained in both the nerve paths described in foregoing paragraphs 

 and enter the spinal cord in the dorsal roots of the lumbar and spinal nerves. 

 Although the anatomic relations of the various nerves composing this mechan- 

 ism are fairly well known, their physiologic actions are not clearly defined. 

 The results of experimental methods of investigation are neither uniform nor 

 in accord. The want of accord lies partly in anatomic peculiarities of the 

 animal the subject of the investigation, and partly perhaps in the character 

 of the stimulus employed. 



Stimulation of the pelvic nerve causes, in the dog at least, a peristaltic 

 contraction of the circular fibers of the rectum. Stimulation of the hypo- 

 gastric nerve causes an inhibition or relaxation of the circular fibers of the 

 rectum and of the internal sphincter as well. Inasmuch as these two groups 

 of fibers have opposite functions it may be assumed that the nerve centers 

 controlling them, both motor and inhibitor, are double centers and that they 

 can be made to act separately and alternately. 



Recalling the events that take place, it may be assumed that peripheral 

 stimulation of the afferent nerves develops nerve impulses which, when 

 transmitted to the cord cause i, a stimulation of the motor center, a dis- 

 charge of nerve impulses through the pelvic nerve to the rectal muscles 

 calling forth a contraction; 2, a stimulation of the inhibitor center, a dis- 

 charge of nerve impulses through the hypogastric nerve to the internal 

 sphincter and perhaps the external sphincter as well, calling forth their 

 relaxation or inhibition. With the discharge of the feces the former condi- 

 tion is re-established. A stimulus, the nature of which is not fully known, 

 causes a stimulation of the inhibitor center for the rectal muscles and a 

 stimulation of the motor center for the sphincters, the nerve impulses reaching 

 the muscles through the hypogastric and pelvic nerves respectively. 



