120 MANUAL OF PHYSIOLOGY. 



tact with the portion of intestine surrounded by this muscle. The 

 rectum for quite one inch above the sphincter is perfectly empty, 

 being kept free from fseculent particles partly by a fold of the in- 

 testinal wall, and partly by the repeated action of the voluntary 

 muscles in the neighborhood, which by intensifying the angle that 

 exists at this point, and flattening this inch of rectum, can squeeze 

 back the approaching matters. Anyone familiar with the digital 

 examination of the unevacuated rectum, knows that no faeces are 

 met with for about two inches. 



Considerable accumulation may take place in the sigmoid flex- 

 ure without much discomfort ensuing, but when the rectum is dis- 

 tended, an urgent sensation of wanting to empty it is experienced, 

 and the voluntary movements mentioned above, are performed by 

 the levator ani and the neighboring perineal muscles, with the 

 object of preventing any substance reaching the part of the rectum 

 immediately above the sphincter. 



If the rectum be distended with fluid, the occasional anal eleva- 

 tion does not suffice to keep it back, and a continuous and com- 

 bined action of the sphincters and levator ani, etc., is necessary to 

 ward off the expulsion of the contents. 



When the lower bowel is habitually emptied at the same hour 

 daily a habit which should be carefully exercised the sensations 

 of requirement to go to stool occur with great punctuality, or can 

 be readily induced by the will, so that normal defecation is re- 

 puted to be, and practically is, a voluntary act. But not com- 

 pletely so, for somewhat like swallowing, the later stages of defse- 

 cation consist essentially of a series of involuntary reflex events 

 which we can initiate by the will, but when it is once started, are 

 powerless to modify until the reflex sequence is completed. 



Under ordinary circumstances, the evacuation of the fieces is 

 commenced by the voluntary pressure exercised on the abdominal 

 contents by the respiratory muscles. The diaphragm is depressed, 

 the outlet of the air passages firmly closed, and the expiratory 

 muscles thrown into action, while at the same moment the muscles 

 which close the pelvic outlet relax, and allow the anus to descend, 

 so that the inferior angle of the rectum is straightened and a vol- 

 untary inhibition of the sphincter is brought about. This volun- 



