36 HISTOLOGY. 



of the epithelium. This divides by branching, and subdivides as shown 

 in the diagram, Fig. 37, A, B, and C. A cavity appears in the cord of cells 

 which then become clearly epithelial. Simple glands, as in the intestine, 

 may remain in the stage A, and be lined throughout with secreting cells; 

 in glands of greater size and complexity only the terminal portions contain 

 the essential secreting cells. The trunk and its main branches serve to 

 convey the products of the "end pieces" to the surface, and constitute. the 

 ducts. Stage B is permanent in such simple glands as those of the stomach, 

 in which a short duct without branches is formed by the union of a few 

 tubular end pieces. The compound glands generally have branching 

 ducts as in C and D. 



The secreting portions of the gland may be tubular, spheroidal, 

 or of some intermediate shape. A round termination is called either an 



acinus (Latin, a grape) or an alveolus (Latin, 

 a small rounded vessel). The intermediate 

 forms are called alveolo-tubular [tubulo- 

 acinar, etc.]. The cavity of these parts is 

 called the lumen of the gland, and is directly 

 continuous with the cavity of the ducts. 

 The secretion may pass from the cells di- 

 rectly into the gland lumen, or it may enter 

 extensions of the lumen found either between 

 the cells or actually within their protoplasm. 

 These are the intercellular and intracellular 

 FIG. 3 8. DIAGRAM OF A SIMPLE AL- secretory capillaries respectively. They may 



VEOLAR GLAND, SHOWING INTER- 

 CELLULAR SECRETORY CAPILLA- be branched or anastomosing, that is, form- 



RIES OR CANALS ON THE RIGHT, 



AND INTRACELLULAR CANALS ON j n g network by the union of their branches. 



THE LEFT. * 



The intracellular capillaries have less distinct 



walls than the others, and are considered transient formations related 

 to vacuoles. The diagram, Fig. 38, represents one half of a simple alveolar 

 gland with intercellular secretory capillaries on the right, and intracellular 

 ones on the left. Both kinds are found in the sweat glands, the liver, 

 and the gastric glands. Intercellular capillaries only are found in the 

 serous glands of the tongue and in the serous portions of the salivary glands, 

 also in the bulbo-urethral, pyloric and lachrymal glands. Secretory capil- 

 laries are apparently absent from mucous, duodenal, intestinal, uterine 

 and thyreoid glands, and from the kidney and hypophysis. 



Having reached the gland lumen, the secretion may pass into a narrow 

 duct lined with simple cuboidal or flat epithelium, the intercalated duct 

 of Fig. 37, D. The transition from this to the larger duct, lined perhaps 

 with columnar epithelium, is not as abrupt as in the diagram. In certain 



