DIGESTION IN THE MOUTH 743 



the sub-lingual gland is wanting, its place being taken by a gland 

 situated somewhat further back and known as the retro-lingual 

 gland. In the pig both retro-lingual and sub-lingual glands are 

 present in addition to the sub-maxillary, and one may sometimes 

 find traces of the retro-lingual gland in man. Many animals, e.g. 

 the dog, also possess a gland situated in the orbit, which pours its 

 secretion into the mouth the orbital gland. These glands can be 

 divided, according to their structure and the nature of the secretion 

 which they form, into several classes. Among the salivary glands 

 of the mucous membrane we may distinguish two types, the mucous 

 gland and the serous gland. In specimens hardened and stained 

 in the ordinary methods the mucous gland is distinguished by the 

 fact that its short duct opens into wide alveoli, the lining cells of 



FIG. 320. A, serous gland ; B, pure mucous gland from mouth. (KOLLIKBR.) 

 a, ducts ; /, fat- cells. 



which are distended with mucin and therefore present a clear 

 unstained space in the section. In the other type, the serous gland, 

 the duct lined with columnar cells branches into a series of acini 

 which present a well-marked lumen and are lined with smal 1 granular 

 cells with a very distinct and well- staining nucleus. The same general 

 distinction can be made out in the large salivary glands. The parotid 

 gland in man and in all the higher mammalia is a typical serous 

 gland, though here and there a mucous cell may be occasionally seen. 

 The orbital gland of the dog represents practically one of the mucous 

 glands of the general mouth cavity on a large scale. The sub-lingual 

 and sub-maxillary glands in man represent a third type. Most 

 of the alveoli are mucous in character. At the ends of the alveoli 

 are seen crescent-shaped cells between the mucin-distended cells and 

 the basement membrane. These are known as the demilune cells, or the 

 crescent cells of Gianuzzi. In some cases these mucous alveoli with 

 demilunes may be found alongside of typical serous alveoli. 



Thus in man the sub -maxillary gland is usually a mixed gland, the serouj 

 alveoli predominating. The sub-lingual gland is also mixed, but with a pre- 

 dominance of the mucous aheoli. In the monkey the sub -maxillary gland 

 is almost entirely serous. In the dog the sub-maxillary gland is a pure mucous 



