THE GLANDS. 



127 



FIG. 166. Portions of salivary glands, showing terminal 

 ducts and secretion-canaliculi ; A, from submaxillarv of 

 dog the canaliculi extend to the demilunes of serous 

 cells; B, from submaxillary of rabbit the canaliculi pass 

 between the serous cells. X 500 and 290. (Ketzius.) 



According to the proportions of the two types of alveoli, the tubo-alveolar 

 glands have been arranged in four groups: (a) pure serous glands, in which 

 only serous alveoli are present, as the parotid; (b} mixed serous glands, in 

 which a few mucous alveoli are 

 intermingled with the serous, as 

 in the submaxillary; (c) mixed 

 mucous glands, in which the 

 serous cells occur as demilunes, 

 as in the sublingual and buccal ; 

 and (d) pure mucous glands, in 

 which only mucous alveoli, with- 

 out demilunes, are found, as in 

 the palatal. 



Simple alveolar glands 

 in their typical flask-like form, 

 abundant in the skin of the 

 lower vertebrates, are repre- 

 sented by the simple sebaceous 

 glands. The dilated sac-like 

 fundus is lined with clear 

 and distended cells, which become modified into duct-cells at the exit. 

 Compound alveolar glands consist of a number of saccular alveoli 

 that open into a common duct, as in the case of the large sebaceous glands 

 and the tarsal glands of the eyelid ; or they may be 



^. ^ * much more complex, being made up of a number 



of alveolar systems, the ducts of which join a large 

 excretory passage. When of such composition they 

 strongly resemble the tubo-alveolar type, the saccular 

 character of the alveoli being the chief distinction. 

 The parotid and the serous part of the submaxillary 

 are regarded by some histologists as examples of 

 compound alveolar glands. The lung affords a con- 

 spicuous example of the principle of the compound 

 saccular type in its mode of development and the 

 arrangement of the air-tubes and saccular terminal 

 compartments. 



The blood-vessels distributed to glands are 

 always numerous, since an adequate blood-supply is 

 necessary to bring to the gland-cells the materials 

 from which their cytoplasm may select the substances 

 required for their metabolism and secretory activity. 

 In the case of the smaller and simpler glands, the 

 capillaries of the mucous membrane form a meshwork 

 I outside the basement membrane enclosing the glan- 

 jf i^~ '> ' dular epithelium. The large compound glands are 



/ "' provided with vessels whose general arrangement 

 FIG.' i6 7 .-injected K a s tric corresponds with that of the duct-system, tin- Mood- 

 mucous membrane, showing vessels following the tracts of interlobular connective 



capillary network surround- . j . , .r \ \- /-\ 



ing tubular glands, x 50. tissue and its extensions between the alveoli. On 

 reaching the latter the capillaries form networks that 



overlie the basement membrane and thus bring the blood-current into close, 

 but not direct, relation with the secreting cells. When the relation between 

 the capillaries and the cells is unusually intimate, as it is in the liver or the 



