1532 



HUMAN ANATOMY. 



FIG. 1286. 



Opening on 

 mucous membrane 



Excretory duct 



exists, some important glands, as the salivary, being in fact a blending of the two 

 types; such glands are, therefore, appropriately termed tubo-alveolar. ..... 



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

 depression lined by epithelium directly continuous with that covering the adjacent sur- 

 face of the mucous membrane, as an outgrowth of which the gland originally devel 

 oped. In such simple gland the two fundamental parts, \tefundus and the duct , are 

 seen in their primary type. The fundus includes the deeper portion of the gland i 

 which the epithelium' has assumed the secretory function, the cells becoming larger and 

 more spherical in form, while in structure the distinction between the spongioplasm and 

 hyaloplasm is usually marked in consequence of the particles of secretion stored up 

 within the meshes of the spongioplastic net-work, which is often sharply displayed. 

 The duct connects the fundus with the free surface and carries off the products elabc 

 rated within the gland. It is lined with cells which take no part in secretion and 

 hence retain for some distance the character of the adjacent surface epitheln 

 Dilatation of the fundus of the primitive type produces the simple alveolar or saccular 



gland ; division of the fundus and part or 

 the duct gives rise to the compound tubu- 

 lar variety ; repeated cleavage and subdivi- 

 sion of the duct, with moderate expansion 

 of the associated terminal tracts, lead to 

 the production of the tubo-alveolar type. 

 Simple tubular glands may be 

 minute cylindrical depressions of practi- 

 cally uniform diameter, as the crypts of 

 Lieberkiihn in the intestine, or they may 

 be somewhat wavy and slightly expanded 

 at the fundus, as often seen in the gastric 

 glands towards the cardiac end of the 

 stomach. When the torsion becomes 

 very pronounced, as in the sweat-glands, 

 the coiled variety results. 



Compound tubular glands pre- 

 sent all degrees of complexity, from a 

 simple bifurcation of the fundus and ad- 

 jacent part of the duct, as in the pyloric 

 or uterine glands, to the elaborate duct- 

 system ending in terminal divisions either 

 of a tubular form, as in the kidney and tes- 

 ticle, or of a modified, somewhat dilated, 

 alveolar form, the tubo-alveolar type, as 

 in the salivary glands. 



Tubo-alveolar glands, modified 

 compound tubular, constitute a very im- 

 portant group, since they embrace many 

 of the chief secretory organs of the body. 

 They are made up by repetition of similar structural units, differences in the size of 

 the organ depending upon the number of those associated to compose the gland. 

 These unit-. < -m respond to the groups of terminal compartments, or alveoli, con- 

 nected with a single ultimate division of the duct-system. The alveoli or acini contain 

 the secreting cells, and arc limited externally by a basement membrane, often well 

 developed, which supports the glandular epithelium and separates the latter from the 

 blood- and lymph-vessels that surround the acinus. 



The alveoli belonging to the same intermediate duct, held together by delicate 

 connei me tissue, constitute a pyramidal mass of glandular tissue, \hz. primary lobules. 

 The latter are assembled into larger groups, or secondary lobules, which in turn are 

 united by interlobular connective tissue into the lobes composing the entire gland. 

 The lol.es .ue held together more or less tirmly by the interlobar areolar tissue 

 continuous with the general fibrous envelope, which forms a capsule for the entire 

 organ and separates it from the surrounding structures. 



Beginning of 

 duct in alveoli 



Terminal 

 alveolus 



Diagram showing relations of various portions of duct- 

 system in glands of tubo-alveolar type. 



