220 THE ORGANS 



forms a more or less definite capsule. From the capsule connective- 

 tissue sepia or trabeculce extend into the gland. The broadest septa 

 usually divide the gland into a number of macroscopic compartments 

 or lobes. Smaller septa from the capsule and from the interlobar 

 septa divide the lobes into smaller compartments usually microscopic 

 in size— the lobules. A lobule is not only a defmite portion of the 

 gland separated from the rest of the gland by connective tissue, but 

 represents a defmite grouping of tubules or alveoli with reference to 

 one or more terminal ducts. The glandular (epithelial) tissue is 

 known as the parenchyma of the gland, in contradistinction to the 

 connective or interstitial tissue. 



The relations of the glandular tissue proper to the connective tissue are best 

 understood by reference to development. All glands, simple and compound, 

 originate as simple evaginations from a surface lined with epithelium. The 

 epithelial evagination grows down into the underlying connective tissue. In a 

 compound gland this invagination tubule becomes the main excretory duct. As 

 the tubule grows, it divides and subdivides to form the larger and smaller ducts 

 and finally the secreting tubules or alveoli. During the development of the 

 gland tubules, the connective tissue is also developing, but is being largely re- 

 placed by the more rapidly growing tubules. The gland tubules do not develop 

 irregularly, but in definite groups, each group being dependent upon the tubule 

 (duct) from which it originates. Thus the invagination tubule (main excretory 

 duct) gives rise to a few large branches (lobar ducts), each one of which gives 

 off the subdivisions which constitute a lobe. From each lobar duct there arise 

 within the lobe a large number of smaller branches (lobular ducts) each one of 

 which gives rise to the subdivisions included in a lobule. As the lobe groups and 

 lobule groups of tubules develop, the largest strands of connective tissue are left 

 between adjacent lobes (interlobar connective tissue), smaller strands between 

 lobules (interlobular connective tissue), and the finest connective tissue between 

 the tubules or alveoli within the lobule (intralobular connective tissue). 



Glands may thus be classified as follows: 

 A. Duct glands or glands of external secretion. 

 I. Tubular glands. 



to^ 



(a) Simple tubular 



straight. 



coiled. 



branched. 



(b) Compound tubular. 

 2. Alveolar or saccular glands. 



(a) Simple alveolar. 



(b) Compound alveolar, saccular or racemose. 

 B. Ductless glands or glands of internal secretion. 



