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 

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



FIG. 110. DIAGRAM SHOWING VARIOUS FORMS OF SBCKETIXG GLA>T>S. 1, gen- 

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

 c, connective 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- 



Is. The simplest form in which a secretory apparatus 

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



