Chap. XIV.] 



ALIMENTATION. 



165 



action, and poured out upon the external or internal surfaces 

 of the body. An excretion resembles a secretion, except that 

 whereas the secretion is formed to perform some office in the 

 Dody, the excretion is formed only to be thrown out of the body. 



Fig. 110. — Diagram showing Various Forms of Secreting Glands. 1, gen- 

 eral plan of a secreting membrane; a, epithelial cells; b, basement membrane; 

 c, coimective tissue in which lie the blood-vessels (d) ; 2-7, simple and compound 

 tubular and saccular glands ; d, duct. 



A secretory apparatus consists essentially of a layer of secret- 

 ing cells placed in close communication with a network of blood- 

 vessels. The simplest form in which a secretory apparatus 

 occurs is in the shape of a plain, smooth surface, composed of 



