196 
DEFECATION-LACTEAL ABSORPTION. 
the surrounding parts ; and it is kept-in by a circular muscle 
or sphincter , which surrounds the outlet of the alimentary 
canal. But when the accumulation has taken place beyond 
this amount, it excites a reflex action (§ 195) in the muscles 
that surround the abdomen; and these make pressure suf¬ 
ficient to overcome the resistance of the sphincter, and to 
force out the contents of the rectum. 
Absorption of Nutritive Material . 
217. We have only now to inquire into the mode, by which 
the nutritive matter extracted from the food is taken-up from 
the alimentary canal and applied to the nutrition of the body. 
In all Yertebrated animals, there exists a special set of vessels 
termed Absorbents; of which those forming one division, 
Thoracic Mesenteric 
Aorta Duct Glands 
Fig. 114.— Chyle-vessels. 
known as Lacteals , from the milk-like character of their con¬ 
tents, originate in the numberless villi or minute projections 
with which the mucous membrane that lines the small intes¬ 
tine is covered (§ 41). During the act of digestion, the 
