GLANDS. 



35 



Besides the cytogenic glands, there are epithelioid glands consisting 

 of clumps or cords of cells resembling epithelium, yet having no free surface. 

 These masses of cells, which may be detached from an epithelium or 

 formed from mesenchyma, are generally penetrated by blood or lymphatic 

 vessels into which their secretions are discharged. Secretions eliminated 

 in this manner are called internal secretions. The epithelioid glands can 

 produce only internal secretions. The suprarenal gland is a large example 

 of this class. 



Epithelial glands are such as consist of true epithelium, discharging 

 their secretions from its free surface. Most glands are of this nature. 

 In simplest form they are merely the occasional mucous^or' other secret- 

 ing cells found scat- 

 tered over an epithe- 

 lium. These are 

 sometimes called 

 unicellular glands. 

 Others are simple 

 tubular or saccular 

 depressions in the 

 epithelium, lined 

 with secreting cells 

 as shown in Fig. 36. 

 Glands of this de- 

 scription, perhaps 

 coiled at their lower 

 end, or having a few 

 branches, or con- 

 sisting of a cluster 

 of saccular secret- 

 ing spaces, often 

 occur in large numbers as parts of some organ. Thus they are found in 

 the intestine, the uterus, and the skin, where they are named intestinal 

 glands, uterine glands, sebaceous and sweat glands respectively, each 

 kind having its special characteristics. They are named as classes and 

 not as individuals, and have been grouped as the simple glands. On the 

 other hand, there are epithelial glands which occur singly or in circum- 

 scribed groups, having their own connective tissue capsule, blood, nerve, 

 and lymph supply. Such forms are considered as separate organs, for 

 example, the liver, pancreas, mammary gland, and prostate, and for this 

 group the name compound glands has been introduced. 



These glands develop in the embryo generally as a solid downgrowth 



Inter- 

 calated 

 duct. 



End pieces. 



FIG. 37. DIAGRAM OF THE DEVELOPMENT OF A COMPOUND GLAND. 

 The arrangement of ducts in D is that of the human submaxillary gland. 



