THE GLANDS. 



123 



tive tissue and provided with ducts and blood-vessels. The application of 

 the term to lymph-nodes is undesirable, since these structures do not secrete. 

 Further, designating such organs as the thyroid, parathyroid and suprarenal 

 bodies as ' ' ductless glands ' ' seems less satisfactory, from the view-point of 

 accurate terminology, than grouping them as "organs of internal secre- 

 tion." Only those organs which produce secretions that are carried off 

 through definite openings or ducts are entitled to the term gland. 



Glands are classified according to their form into two chief groups, the 

 tubular and the alveolar, each of which occurs as simple or compound. In 

 many instances, however, no sharp distinction between these conventional 



FIG. 162. Diagram illustrating types of glands, a-e, tubular ; f-i, alveolar or saccular. a, simple ; 6, 

 coiled ; c, d, increasingly complex compound tubular; e, tubo-alveolar ; f, simple; g, h, i, progressively 

 complex compound alveolar. 



groups exists, some important glands, as the salivary, being in fact a 

 blending of the two types; such glands are, therefore, appropriately called 

 tubo-alveolar. 



In the least complex type, the simple tubular, the gland consists of a 

 cylindrical depression lined with epithelium continuous with and covering 

 the adjacent surface of the mucous membrane, as an outgrowth of which it 

 is developed. In such simple glands the two fundamental parts, \he fundus 

 and the duct, are seen in their primary type. The fundus includes the 

 deeper portion of the gland, in which the epithelium has assumed the secre- 

 tory function, the cells becoming larger and more spherical. The distinction 

 between spongioplasm and the intervening substances is usually marked in 

 consequence of the particles of secretion (metaplasm) stored up within the 

 meshes of the spongioplastic reticulum; hence the latter is often strikingly 

 displayed. The duct connects the fundus with the free surface and carries 

 off the secretion produced by the gland-cells. It is lined with cells that take 

 no part in secretion and retain for some distance the character of the adjoin- 



