CHAPTEE XXX 



SALIVA 



THE saliva is formed by three pairs of salivary glands, called the 

 parotid, submaxillary, and sublingual glands. 



The Salivary Glands. 



These are typical secreting glands. They are made up of lobules 

 united by connective tissue. Each lobule is made of a group of tubulo- 

 saccular alveoli or acini, from which a duct passes ; this unites with 



other ducts to form larger and larger 

 tubes, the main duct opening into the 

 mouth. 



Each alveolus is surrounded by a 

 plexus of capillaries ; the lymph which 

 exudes from these is in direct contact 

 with the basement membrane that en : 

 closes the alveolus. The basement mem- 

 brane is lined by secreting cells which 

 surround the central cavity or lumen. 

 The basement membrane is thin in 

 many places, to allow the lymph more 

 ready access to the secreting cells ; it 

 is continued along the ducts. 



The secreting epithelium is com- 

 posed of a layer of polyhedral cells. 

 The epithelium of the ducts is columnar, except where it passes 

 into an alveolus ; at this point it is flattened. The columnar epithelium 

 cells of the ducts exhibit striations in their outer part (see fig. 393) ; 

 the inner zone of each cell is made of granular protoplasm. The 

 largest ducts have a wall of connective tissue outside the basement- 

 membrane, and a few unstriated muscular fibres. 



The secreting cells differ according to the substance they secrete. 

 In alveoli that secrete mucin (such as those in the dog's submaxillary, 



re. 



FIG. 393. From a section through a 

 salivary gland, a, serous or albumi- 

 nous alveoli ; b, intralobular duct 

 cut transversely. (Klein and Noble 

 Smith.) 



474 



