DIGESTION OF POOD. 339 



What happens during an ordinary act of defecation is ahout 

 as follows : After a long inspiration the glottis is closed ; the 

 diaphragm, which has descended, remains low, affording, with 

 the obstructed laryngeal outlet, a firm basis of support for the 

 action of the abdominal muscles, which, bearing on the intes- 

 tine, forces on their contents, which, before the act has been 

 called for, have been lodged mostly in the large intestine ; at 

 the same time the sphincter ani is relaxed and peristaltic move- 

 ments accompany and in some instances precede the action of 

 the abdominal muscles. The latter may contract vigorously on 

 a full gut without success in the absence of the intestinal peri- 

 stalsis, as too many cases of obstinate constipation bear witness. 



Like deglutition, and unlike vomiting, there is usually both 

 a voluntary and involuntary part to the act. 



Though the will, through the cerebrum, can inhibit defeca- 

 tion, it is likely that it does so through the influence of the 

 cerebrum on some center in the cord ; for in a dog, the lumbar 

 cord of which has been divided from the dorsal, the act is, like 

 micturition, erection of the penis, and others which are under 

 the control of the will, still possible, though, of course, per- 

 formed entirely unconsciously. 



Vomiting. — If we consult our own consciousness and observe 

 to the best of our ability, supplementing information thus 

 gained by observations on others and on the lower animals, it 

 will become apparent that vomiting implies a series of co-ordi- 

 nated movements into which voUtion does not enter either 

 necessarily or habitually. There is usually a preceding nausea, 

 with a temporary flow of saliva to excess. The act is initiated 

 by a deep inspiration, followed by closure of the glottis. 

 Whether the glottis is closed during or prior to the entrance 

 of air is a matter of disagreement. At all events, the dia- 

 phragm descends and remains fixed, the lower ribs being re- 

 tracted. The abdominal muscles then acting against this sup- 

 port, force out the contents of the stomach, in which they are 

 assisted by the essential relaxation of the cardiac sphincter, the 

 shortening of the oesophagus by its longitudinal fibers, and the 

 extension and straightening of the neck, together with the open- 

 ing of the mouth. 



As the expulsive effort takes place, it is accompanied by an 

 expiratory act which tends to keep the egesta out of the larynx 

 and carry them onward, though it may also contribute to over- 

 come the resistance of the elevated soft palate, which serves to 



