EPITHELIUM. 51 



larger ducts there is a quite strongly developed layer of smooth 

 muscle. 



Glands are among the most richly vascular tissues. The 

 blood-vessels divide into fine capillaries, which surround the 

 tubuli or alveoli, and run along the basal ends of the gland 

 cells. The blood flowing to the gland carries with it materials 

 used in the formation of the secretion, the gland cells being 

 between the blood-vessels and the lumen. The constituents of 

 the secretion may be directly taken up from the blood ; but 

 usually they are the result of specific metabolic changes in the 

 gland cells, some materials being, however, supplied by the 

 blood. Also the secretion may have partly one and partly the 

 other origin. 



In some glands the secretion proceeds to the lumen not only 

 from the surface of the cells, but also through fine canals, the 

 so-called secretory capillaries, it is carried in all directions (see 

 Salivary Glands and Stomach). These secretory capillaries, 

 which anastomose freely with one another to form a sort of 

 network, open finally into the gland lumen. 



The materials which are secreted internally are taken up 

 by the blood and carried to the parts of the body in which 

 they are used. 



The varied appearances met with in gland cells (granular, 

 vacuolated, striated, etc.) are due in large part to the kind of 

 secretion present. This may be equally varied, such as mucus, 

 bile, urine, gastric juice, ferments, sugar, etc. Likewise the 

 appearance of the secretory cell changes according to its degree 

 of activity. There may be various stages of functional activity 

 shown at the same time in the cells of a tubule or alveolus. 

 Some are filled with materials which they are about to secrete, 

 while others are shrunken and empty on account of having 

 discharged their products. 



Chorda Dorsalis. 



The tissue of the chorda dorsalis occupies an uncertain 

 position in the classification. This structure is present only in 

 the embryonic life of higher vertebrates, and is made up of 



