CHAP. VIIT.] ORGANS OF KEtiPIBATION AND SECRETION. 231 



and mucous glands, and complex ones in the salivary glands, the 

 pancreas, and above all, the liver. The foldings or subdivisions of 

 a gland are manifestly but a convenient mode of augmenting the 



space. Since all secretions are 



secreting surface within a small 





Fig. 103. DIAGRAM OF DIFFERENT FORMS OF GLANDS, SHOWING HOW THE SECRETING SURFACE 



MAY BE AUGMENTED, AND THE GLANDULAR STRUCTURE RENDERED MORE COMPLEX BY 



INVERSION OR RECESSION OF THE SURFACE TO A GREATER AND GREATER DEGREE. 



A. Simple glands. 



g. Straight tube. 



h. Sac. 



i. Coiled tube. 



B. Slightly more complex forms. 



k. Tubular. 

 I. Saccular. 



C. Racemose or compound saccular gland 



in. The entire gland, showing its branching 



duct and lobular structure. 

 n. A separated lobule, with one branch of 



the duct (o) proceeding from it. 

 D. Compound tubular gland. 

 Secreting surfaces may also be increased by 

 projections and foldings outwards, analogous 

 to these inversions and foldings inwards. 



formed from the blood, every secreting surface, and therefore every 

 gland, must be highly vascular. It is as yet quite unknown what 

 causes different epithelial cells to have the power of forming such 



