GLANDS 



253 



GLANDS 



The secreting glands may he quite properly considered as invagina- 

 tions of the epithelial surfaces of the mucous membranes. They appear 

 as such in the embryo. Their earliest anlage is formed by a solid or 

 funnel-shaped process of epithelium, in which a distinct lumen soon 

 appears, and which grows into the surrounding mesoblast, carrying with 

 it its embryonal tunica propria. 



CLASSIFICATION OF GLANDS 



The form of the glandular invaginations is subject to great variation. 

 They may be straight and simple, more or less branched and compound, 

 convoluted or coiled, or the tubules may terminate in minute ampullary 



FIG. 255. DIAGRAMS OF THE PRINCIPAL TYPES OF GLANDS. 



T, simple tubular (intestinal glands); Ts, slightly coiled tubular (intestinal glands 

 and small sweat glands) ; Tss, more extensively coiled tubular (larger sweat glands) ; 

 Tr, branched tubular (gastric glands) ; compound tubular (kidney, lacrimal glands) ; 

 As, simple alveolar or saccular (small sebaceous glands) ; Ar, branched alveolar (larger 

 sebaceous glands, tarsal glands of eyelid); Ac, compound tubulo-alveolar or racemose 

 (A) (salivary glands); compound saccular (B) (mammary gland). In major part after 

 Prevant, Brown and Maillard. 



enlargements, the acini or alveoli. The invaginations may also be dis- 

 tinctly tubular and of approximately equal diameter throughout, or 

 they may form pouch-like saccules. According to the form of the organ, 

 it is thus possible to distinguish the following 



