DEFECATION. 



267 



of the cord, and from this impulses are reflected which, conveyed 

 to the sphincter, cause its relaxation. Under the influence of 

 similar impulses which pass to the levator ani this muscle con- 

 tracts and draws upward the edges of the anus, causing it to open, 

 while at the same time the muscular fibers of the rectum contract 

 and expel the feces. If the stimulation is very pronounced, the 

 abdominal muscles may also be called into action irrespective of 

 the will, but when the stimulus is slight they may only respond 

 when called upon by the brain. The connection between the brain 

 and the anospinal center is very close, so that the action of the 

 latter may for a time be inhibited ; but if the rectum becomes very 

 much distended, the impulses may be so strong that, despite the 

 will, defecation will take place. 



FIG. 141. Microscopic constituents of the stools: a, vegetable fragments; 6, 

 muscular fibers; c, white blood-corpuscles; d, saccharomyces; e, micro-organisms ; 

 /, crystals of triple phosphate; g, fatty-acid crystals (partly from Jaksch). 



FIG. 142. Monads from the feces : a, Trichomonas intestiualis ; 6, Cercomonas in- 

 testinalis ; c, Ameba coli ; d, Paramcecium coli ; e, living monads ; /, dead monads 

 (Jaksch). 



Involuntary Discharges. In some forms of disease the irrita- 

 bility of the anospinal center is so great that when the rectum is 

 only partially filled defecation takes place, and there is no power 

 to retard it.* Discharges under these circumstances are said to 

 be involuntary. 



