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. 
Tn all Vertebrated animals, there exists a special set_of vessels 
termed Absorbents; of which those forming one division, 
Thoracic Mesenteric 
Aorta Duct Glands 
Origins of 
Lacteal Vessels 
Lymphatic Mesentery 
Vessels 
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 
