302 



COMPARATIVE ANATOMY 



The sublingual is also a mixed tubulo-acinous gland lying below the 

 tongue in the front of the mouth near the median line. Mucus and serous 

 cells are about evenly distributed. In each acinus, the mucus cells are 

 central and the serous peripheral so that the serous tend to form demi- 

 lunes. The openings of the sublingual ducts lie in front of the tongue near 

 those of Wharton's ducts. 



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

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

 origin. This is probably not true, however, of the numerous glands of 



A crescent consisting of 

 eight serous cells. 



Part of an excretory duct 



<4' 



Lumen, 



Tangential 

 section of serous 

 cells. 



Mucous cells and 



thick membrana 



propria . 



Fig. 253. 



-Section of a human sublingual gland X252. 

 Histology.") 



(From Bremer's "Text Book of 



the tongue, all of which are formed by the local prolification of the stratum 

 germinativum of the 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 salivary glands of inverte- 

 brates have any genetic relation with those of vertebrates. 



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 amphibia have an intermaxillary gland, 



