DEFECATION. 



315 



faeces may not excite reflexly the sphincter-muscles, in consequence of 

 mechanical stimulation of the sensory nerves of the rectum, there seems 

 to be an inhibitory centre for the reflex action of the sphincters, which is 

 set in action, owing, as it appears, to voluntary impulses. Its seat 

 is in the brain; Masius thinks it is in the optic thalami, from whence 

 fibres pass through the peduncles of the cerebrum to the lumbar 

 part of the spinal cord. When this inhibitory apparatus is in action, 

 the faecal mass passes through the anus, without causing it to close 

 reflexly. 



The strong peristalsis which precedes defsecation can be aided, and to 

 a certain degree, excited by voluntary, short, movements of the external 

 sphincter and levator ani, whereby the plexus myentericus of the large 

 intestine is stimulated mechanically, thus causing lively peristaltic 



Fig. 130. 

 Levator ani and Sphincter ani externus. 



movements in the large intestine. The expulsion of the fasces is also 

 aided by the pressure of the abdominal muscles, and most efficiently 

 when a deep inspiration is taken, so as to fix the diaphragm, whereby 

 the abdominal cavity is diminished to the greatest extent. The soft 



