CHAP. XIII.] 



ALIMENTATION. 



143 



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. 



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 appa- 



FIG. 93. DIAGRAM SHOWING THE VARIOUS FORMS OF SECRETING STRUCTURES. 

 A, general plan of a secreting membrane ; 6, basement membrane, with cells (a) on one 

 side and blood-vessels (c) on the other; B and E, simple tubular, saccular, and 

 coiled tubular glands ; C, compound tubular gland ; D, compound saccular, or race- 

 mose gland. 



ratus occurs is in the shape of a plain, smooth surface, com- 

 posed of a single layer of epithelial cells, resting usually on a 

 thin membrane, on the under surface of which is spread out a 

 close network of blood-vessels. In order to economize space 

 arid to provide a more extensive secreting surface, the mem- 

 brane is generally increased by dipping down and forming vari- 

 ously shaped depressions or recesses, these depressions or recesses 

 being called the secreting glands. 



The secreting glands are of two kinds, simple and compound. 

 The simple glands are generally tubular or saccular cavities, the 

 tube in the tubular variety being sometimes so long that it coils 

 upon itself, as in the sweat glands of the skin ; they all open 

 upon the surface by a single duct. In the compound glands, 

 the cavities are subdivided into smaller tubular or saccular 

 cavities, opening by small ducts into the main duct which pours 

 the secretion upon the surface. 



However simple or complicated the involuted surface, the 

 secreting process is essentially the same ; and in this process 

 the nucleated cells play the most important part. These cells 

 take into their interior those substances from the blood which 

 they require to make the special secretion they are set apart to 



