222 



CHORDATE ANATOMY 



Development. From their position and the relations of their ducts, 

 it is generally assumed that the chief saHvary glands are of ectodermal 

 origin. The numerous glands of the tongue, however, are formed by 

 the local prolification of the stratum germinativum of the endodermal 

 mucous lining of the mouth. 



History of Salivary Glands. Salivary glands are not unknown among 

 the invertebrates. Multicellular mucus glands connected with the 

 mouth are present in molluscs. Malaria is transmitted by the saliva of 

 mosquitoes. It is doubtful, however, if the saUvary glands of inverte- 

 brates have any genetic relation with those of vertebrates. 



Part of an excretory duct 



A crescent consisting of 

 eight serous cells. 



Lumen 



Fig. 209. — Section of a human sublingual gland, X252. 



Histology.") 



Tangential 

 section of serous 

 cells. 



Mucous cells and 



thick membrana 



propria . 



Connective 



tissue. 



(From Bremer's "Text Book of 



Lower chordates have no salivary glands, and fishes only unicellular 

 mucus glands. It has generally been assumed that the multicellular glands 

 of the higher vertebrates have their beginnings in such unicellular glands. 



Multicellular oral glands appear in Amphibia. Besides the mucus- 

 secreting cells of the tongue, most amphibians have an intermaxillary gland, 

 the duct of which opens between the intermaxillary bones. In some 

 amphibians, e.g., Rana, mucus glands are located also in the posterior nasal 

 passages. That enzymes are secreted by the mucus cells of fishes and 

 amphibians has, however, not been demonstrated. 



In the reptiles, there are serous cells in the oral glands, and lingual, 

 sublingual, and palatine glands occur. Glands connected with the teeth 

 are differentiated as the poison glands of some snakes. 



