338 ANIMAL PHYSIOLOGY, 
wholly due to the action of extrinsic muscles; at all events any 
one may convince himself that the latter may be made to take 
a great part in preventing fecal discharge, though whether the 
tone of the sphincter can be increased or not by volition it is 
difficult to say. 
What happens during an ordinary act of defecation is about 
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, performed 
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 volition 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 esophagus by its longitudinal fibers, and the 
extension and straightening of the neck, together with the open- 
ing of the mouth. 
