GLANDS. 1535 



formerly thought to be the case, since extensions of the lumen pass between the 

 mucous cells to reach the demilunes. 



In addition to the main alveolar lumina, always narrow in serous and wider in 

 mucous acini, the existence of intercellular passages, or secretion-capillaries, has been 

 established for many glands, especially by the employment of the Golgi and other 

 special methods. These clefts penetrate laterally be- 

 tween the glandular epithelium from the axial lumen I28 9- 

 towards the basement membrane, partially enclosing 

 the secreting cells with a branching system of minute 

 canals. Alveoli containing exclusively mucous cells 

 do not possess these intercellular canaliculi, the axial 

 lumen alone being present. In acini of the serous 

 type the accessory channels are represented by minute 

 branching passages which penetrate between the cells, 

 but seldom reach the basement membrane. The most 

 conspicuous of the secretion-capillaries occur in alve- 

 oli containing the demilunes, the product of the 

 serous cells escaping into the main lumen by means 

 of the lateral intercellular canals which pass between 

 the mucous elements to reach the peripheral group of Section of several alveoli of submax- 

 serous cells composing the crescent. The view that SJg ftli8g8&?2gS 

 the secretion-capillaries normally extend into the cyto- ceiis^x"^ (s $?!?i*/) roilps * serous 

 plasm of the glandular epithelium, and are, therefore, 



also intracellular, must be regarded as doubtful and still undecided, although sup- 

 ported by many able histologists. 



Depending upon the distribution of the two varieties of alveoli, the tubo- 

 alveolar glands may be divided into four groups (Ebner): 



1 . Pure serous glands, in which only serous alveoli occur, as the parotid. 



2. Mixed serous glands, in which a few mucous alveoli are intermingled with the 

 serous, as the submaxillary. 



3. Mixed mucous glands, in which the serous cells occur as crescentic groups or 

 demilunes, as the sublingual and buccal. 



4. Pure mucous glands, without serous alveoli or demilunes, as the palatal. 

 Simple alveolar or saccular glands in their typical flask-like form, as seen 



in the skin of amphibians, are not found in man. The dilated spherical fundus is lined 

 with clear and distended secreting cells, in which the nuclei are displaced towards the 

 periphery by the mucus elaborated within the epithelial elements. In the higher 

 animals this type of gland is represented, somewhat modified, by the simple sebaceous 

 follicles. 



Compound alveolar or saccular glands constitute a group much less exten- 

 sive than formerly supposed, since careful study of the form and arrangement of many 

 organs, as the salivary glands, pancreas, etc. , has shown that these are more appro- 

 priately regarded as tubo-alveolar than as branched saccular glands. The latter, 

 however, still have representatives in the larger sebaceous and Meibomian glands. 

 The most conspicuous example of the compound saccular or racemose type is the 

 lung, which in its development and the arrangement of the air-tubes and the sac-like 

 terminal compartments corresponds to this variety. 



The blood-vessels distributed to glands are always numerous, since secretory 

 activity implies a generous blood-supply. In the case of the smaller and simpler 

 glands, the capillaries within the mucosa form a mesh-work outside the basement 

 membrane enclosing the glandular epithelium. The large compound glands are pro- 

 vided with a vascular system which usually corresponds in its general arrangement 

 to that of the excretory ducts, following the tracts of the interlobar and interlobular 

 areolar tissue and its extensions between the groups of the alveoli. On reaching the 

 individual acini, the capillaries form net-works which surround the basement mem- 

 brane enclosing the alveoli, thus bringing the blood-current into close, but not direct, 

 relation with the secreting cells, an arrangement favoring the selection by the proto- 

 plasm of the particular substances required for the function of the gland. When the 

 relation between the glandular epithelium and the capillaries is unusually intimate, 



