I2 4 



NORMAL HISTOLOGY. 



ing surface epithelium. Dilation of the fundus of the primitive type produces 

 the simple alveolar or saccular gland; division of the fundus and part of the 

 duct gives rise to the compound tubular variety; repeated subdivision of the 

 duct, with moderate expansion of the associated terminal parts, leads to the 

 production of the tubo-alveolar type. 



Simple tubular glands may be minute cylindrical depressions of 

 almost uniform diameter, as the crypts of Lieberkiihn in the intestine, or 

 they may be somewhat wavy and slightly expanded at the fundus, as seen in 

 the gastric glands towards the cardiac end of the stomach. When torsion 

 becomes very pronounced, as in the sweat-glands, the coiled gland results. 

 Compound tubular glands present all degrees of complexity, from a 

 simple bifurcation of the fundus and adjoining part of the duct, as in the 

 pyloric glands, to the elaborate duct-system ending in tubular terminal 



divisions, conspicuously exampled 

 in the kidney. 



Tubo-alveolar glands, mod- 

 ified compound tubular, constitute 

 a very important group comprising 

 many of the chief secretory organs 

 of the body, as the salivary glands. 

 They are made up by the repetition 

 of similar structural units, differences 

 in the size of the organs depending 

 upon the number of associated units. 

 Each unit corresponds to the groups 

 of terminal compartments, or alveoli, 

 connected with a single ultimate 

 division of the duct-system. The 

 alveoli or acini contain the secreting 

 cells and are limited externally by 

 a basement membrane, often well 

 developed, which supports the glan- 

 dular epithelium and separates the 

 latter from the perialveolar blood- 

 vessels that surround the alveolus. 

 The alveoli belonging to the same 

 terminal duct are held together by 

 delicate connective tissue and constitute a pyramidal mass of glandular tissue, 

 the primary lobule. The primary lobules are assembled into larger groups, 

 the secondary lobules, which in turn are united by the interlobular connective 

 tissue as the lobes of the gland. The lobes are held together more or less 

 firmly by the interlobar areolar tissue continuous with the general fibrous 

 envelope or capsule, which invests the entire organ and separates it from the 

 surrounding structures. 



The interlobar tissue and its interlobular continuations contain the blood- 

 vessels, lymphatics and nerves supplying the gland and, in addition, the major 

 part of the system of duct-tubes. In the larger glands the ducts constitute an 

 elaborate system of passages arranged after the general plan shown in the 

 accompanying diagram (Fig. 163). Traced from the alveoli, the duct- 

 system begins as a narrow canal, the intermediate duct, lined with low 

 cuboidal or flattened cells directly continuous with the glandular epithelium 

 of the alveoli. After a short course the intermediate tube increases in 

 diameter and becomes the intralobular duct, which is often conspicuous on 



FIG. 163. 



Opening on 

 mucous membrane 



Main duct 



Interlobar duct 



- Interlobular duct 



Intralobular duct 



Intermediate duct 



Beginning of duct 

 Terminal alveolus 



is of duct- 



