208 TEXT-BOOK OF PHYSIOLOGY 



there is a return to the former condition, namely, a relaxation or inhibition 

 of the rectal muscles and a contraction of the sphincter muscles. If the act of 

 defecation is to be suppressed, the controlling influence of the nerve-center 

 on the contraction of the external sphincter may by an act of volition be 

 strengthened and the action of the reflex mechanism for a while antagonized. 

 The nerve mechanism therefore involves both efferent and afferent nerves 

 as well as nerve centers in the lumbo-sacral region of the spinal cord. 



Efferent Nerves. The efferent nerve-fibers for the external sphincter mus- 

 cle have their origin in the spinal cord from which they pass by way of the 

 third and fourth sacral nerves, the pelvic nerve and the inferior hemorrhoidal 

 nerve directly to the muscle. 



The efferent nerve-fibers, for the longitudinally and circularly arranged 

 muscle-fibers of the rectum, including the specialized portion, the internal 

 sphincter, have their origin in nerve-cells in the lumbo-sacral region of the 

 spinal cord and pass to their destination by two paths. The fibers in the 

 first path leave the spinal cord by way of the second to the fifth lumbar 

 nerves, then pass into and through the sympathetic chain, through the inferior 

 splanchnics to the inferior mesenteric ganglion around the cells of which 

 their terminal branches arborize; from the cells of this ganglion new fibers 

 emerge which pass through the hyppgastric nerves to the muscles. The 

 fibers of the second path leave the spinal cord by way of the second to the 

 fourth sacral nerves, then pass into the pelvic or erigens nerve to small 

 ganglia along the sides of the rectum around the cells of which their 

 terminal branches arborize; from the cells of these ganglia new nerve- 

 fibers emerge which pass directly to the muscles. In both paths the nerves 

 coming from the cord are pre-ganglionic, those coming from the ganglia, 

 post-ganglionic. 



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 Nerves. The afferent nerve-fibers that excite the central 

 mechanism to activity are contained in the dorsal roots of the spinal nerves. 

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

 ism are fairly well known, their physiologic actions are n6t 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 peristalic 

 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- 



