DIGESTION 211 



The ileo-cecal valve seems to be quite competent for preventing the 

 backward passage of solid matters, that is, into the ileum, but it readily 

 allows liquids to pass in this direction. It is possible then to inject nutrient 

 enemata into the small intestine. Katz and Winkles find that stimulation 

 of the vagus closes the valve, and that the splanchnics influence it to 

 open; stimulation of the hypogastrics seemed to have no effect. (See 

 Absorption, p. 216.) 



Defecation is the process by which the useless residue of the chyle is 

 periodically voided from the gut. In the human animal the process 

 occurs usually once a day, most easily just after the first meal, the neural 

 mechanism being then stimulated by the processes in the upper end of the 

 alimentary canal.. In many cases, however, defecation takes place twice 

 daily, and in a much smaller proportion of persons only every other day. 

 Defecation is a complicated reflex act voluntarily started and inhibitable 

 at any stage. The outer, lower sphincter and the levatores ani muscles 

 supporting the internal sphincter are made of cross-striated fibers. The 

 internal sphincter and the strong circular fibers of the rectal wall and of 

 the sigmoid flexure are of "unstriated" or smooth muscular tissue. It is 

 probable, however, that the fibers of the external sphincter are made of 

 neither typically skeletal nor smooth muscle, for they are unaffected 

 by curare, and, as Goltz and Ewald have shown on dogs, destruction of the 

 cord which causes atrophy of the skeletal muscles leaves the external 

 sphincter unaffected. 



The two sphincters are normally in a state of tonus dependent on 

 their connection with the central nervous system, a tonus that is not 

 destroyed by curare. 



When defecation is to take place, the diaphragm is first made rigid in 

 contraction, and the glottis is shut. The abdominal muscles then unite 

 in forcible expiratory contraction, and, the perineal muscles and the 

 sphincters being relaxed, feces are forced out of the sigmoid flexure 

 (their habitual reservoir normally) into and through the rectum and out 

 at the anus. Powerful contraction of the levatores ani serve, finally, to 

 empty the lower end of the rectum between the two sphincters, which 

 then at once recover their normal closure-tonus. The nerve-center 

 coordinating the various movements of defecation is not as yet certainly 

 known, although Frank-Hochwart has recently found much evidence 

 that in dogs at least it is often to be demonstrated in the posterior end 

 of the posterior central gyrus of the brain, in apes called "Sherrington's 

 center." There are probably subsidiary centers in the lumbar cord or 

 in the ganglia of the pelvis. Defecation is an excellent example of a 

 process once wholly reflex (distention of the sigmoid flexure or irritation 

 of upper end of rectum furnishing the afferent impulses), which has 

 become more or less under the will's control in man and in some of the 

 lower animals long associated with him. In infants and in most brutes 

 the process is still wholly involuntary and largely reflex, as it is also in 

 many conditions of nervous disease. 



