FOOD ' AND DIGESTION. 407 



2C 3 H 6 3 = CnHeO, -f- 2CO 9 + 2H 9 

 Lactic Acid. Butyric Acid. 



Source of the Carburetted Hydrogen. From the decomposition of 

 acetates and lactates and from cellulose (C 6 HIO 5 -j- H 2 = 3 COg + 

 3 CH 4 ). 



Source of the Nitrogen. The nitrogen is derived from the swallowed 

 air. 



Defaecation. 



The act of the expulsion of faeces is in part due to an increased reflex 

 peristaltic action of the lower part of the large intestine, namely of 

 the sigmoid flexure and rectum, and in part to the more or less volun- 

 tary action of the abdominal muscles. In the case of active voluntary 

 efforts, there is usually, first an inspiration, as in the case of coughing, 

 sneezing, and vomiting; the glottis is then closed, and the diaphragm 

 fixed. The abdominal muscles are contracted as in expiration ; but as 

 the glottis is closed, the whole of their pressure is exercised on the ab- 

 dominal contents. The sphincter of the rectum being relaxed, the evac- 

 uation of its contents takes place accordingly; the effect being, of course, 

 increased by the peristaltic action of the intestine. As in the other 

 actions just referred to, there is as much tendency to the escape of the 

 contents of the lungs or stomach as of the rectum; but the pressure is 

 relieved only at the orifice, the sphincter of which instinctively or in- 

 voluntarily yields. 



Nervous Mechanism. The anal sphincter muscle is normally in a 

 state of tonic contraction. The nervous centre which governs this con- 

 traction is probably situated in the lumbar region of the spinal cord, in- 

 asmuch as in cases of division of the cord above this region the sphincter 

 regains, after a time, to some extent the tonicity which is lost immedi- 

 ately after the operation. By an effort of the will, acting through the 

 centre, the contraction may be relaxed or increased. In ordinary cases 

 the apparatus is set in action by the gradual accumulation of faeces in 

 the sigmoid flexure and rectum, pressing by the peristaltic action of 

 these parts of the large intestine against the sphincter, and causing by 

 reflex action its relaxation; this sensory impulse acting through the 

 brain and reflexly through the spinal centre. At the same time that 

 the sphincter is inhibited or relaxed, impulses pass to the muscles of the 

 lower intestine increasing their peristalsis, and, if necessary, to the ab- 

 dominal muscles as welL The action of the centre is therefore double. 



