496 



THE ALIMENTARY CANAL; SECRETING GLANDS [CH. XXXI. 



Every secreting apparatus consists essentially of a layer of secret- 

 ing cells arranged round a central cavity ; they take from the lymph 

 which bathes them the necessary material, and transform it into the 

 secretion which they pour at high pressure into the cavity. 



In the case of the glands concerned in the formation of the 

 various digestive juices, the most important material in the secretion 

 is an enzyme or enzymes. In the cells which form the enzyme, it is 

 first present in the shape of a pro-enzyme or zymogen. The trans- 

 formation of this mother-substance may occur before or during 

 secretion, as is the case for ptyalin, the salivary enzyme ; or after 



PIG. 345. Transverse section through four 

 crypts of Lieberklihn from the large 

 intestine of the pig. They are lined 

 by columnar epithelial cells, the 

 nuclei being placed in the outer part 

 of the cells. The divisions between 

 the cells are seen as lines radiating 

 from L, the lumen of the crypt ; o, 

 epithelial cells, which have become 

 transformed into goblet cells, x 350. 

 (Klein and Noble Smith.) 



FIG. 346. A gland 

 of Lieberkiihn in 

 longitudinal sec- 

 tion. (Brinton.) 



secretion, as is the case for trypsin, one of the most important of the 

 pancreatic enzymes. 



Secreting glands may be classified as follows : 



1. The simple tubular gland (A, fig. 347), examples of which are 

 furnished by the crypts of Lieberkiihn in the intestinal wall. To 

 the same class may be referred the elongated and tortuous sudoriferous 

 or sweat glands. 



2. The compound tubular glands (D, fig. 347) form another 

 division. These consist of main gland-tubes, which divide and 

 subdivide. 



3. The racemose glands are those in which a number of vesicles 

 or acini are arranged in groups or lobules (c, fig. 347). The Meibo- 

 mian follicles of the eyelids are examples of this kind of gland. 

 Some glands, like the pancreas, are of a mixed character, combining 

 some of the characters of the tubular with others of the racemose 

 type ; these are called tubulo-racemose or tubulo-acinous glands. These 



