THE DIGESTIVE APPARATUS AND NUTRITION 227 



descending parts. The rectum is the final portion of the 

 alimentary canal ; it opens externally at the anus. 



The walls of the large intestine are like those of the 

 small intestine, except that the valvulse conniventes and the 

 villi are wanting. In the caecum and colon the longitudi- 

 nal muscular fibers are for the most part collected in three 

 bands, which, being shorter, from end to end, than the other 

 coats, draw up the intestinal wall into puckers, or folds. 



The muscular coat of the rectum is much thicker than 

 elsewhere, and at the anus is a strong band called the 

 internal sphincter muscle. 



320. Secretion. All the living cells of the body are 

 engaged in taking from the blood certain substances suit- 

 able for their own special purposes, and returning to the 

 blood those particles of matter which have fulfilled their 

 mission and are no longer of use. Every cell requires 

 oxygen, and oxidation is now 



understood to occur within 

 all the cells. But certain 

 cells or groups of cells take 

 up also other substances from 

 the blood, and manufacture 

 within themselves a new 

 product having a special 

 function. This process is 

 called secretion, and the or- 

 gans of secretion are called 

 glands. 



321. Glands. The sim- 

 plest glands are merely 

 minute tubes lined with cu- 

 bical cells (Fig. 114). Sometimes these tubes branch at 

 the inner end, all the branches being lined with the secret- 



Fig. 114. Structure of glands. 



1 simple pit, surrounded by capillaries. 



2 flask-shaped gland, with short duct. 

 3, 4 more complex glands, with longer 



ducts. 



