168 



BIOLOGY. [Book ii. 



very analogous to each other, secrete salivas of diverse species. 

 In general the salivary glands are wholly lacking in the aquatic 

 animals. 



In the salivary glands, the organs of secretion, the glandular ' 

 elements are agglomerated ; but in the stomach of the superior 

 vertebrates these elements are separated and hidden in the 

 substance of the mucous membrane. Their number, moreover, 

 is immense ; they are blind glands and separated from each other 

 only by a thin layer of cellular tissue. These small glands are 

 related and proportioned to the numerous capillary vessels, and 

 to the nervose thi-eads. Besides the glandules of the stomach 

 there are many others, scattered over the whole extent of the 

 digestive canal. They assume variable forms : sometimes they 

 are simple depressions, small purses, small tubes : sometimes they 

 ramify. 



But the most important glandular appendix of the digestive 

 canal of the vertebrates is assuredly the liver. 



In the most inferior vertebrate, the one which in a certain 

 measure connects the vertebrates with the moUusks, the am- 

 pMooeus, the liver is represented only by a caecum of a greenish 

 tint, reminding us of the simple cvls-de-sac, short, not ramified, 

 which seem to play the part of liver in the invertebrates. It is 

 also under an analogous form, under the form of a pointed cone, 

 that the liver originates in reptiles, birds, and mammifers.^ In 

 the superior mammifers, the liver — ^very voluminous — is, abstrac- 

 tion being made of the cellular tissue, essentially constituted by 

 vessels and nerves, first of all by special cells, charged, as we 

 have seen, to fabricate a glycogetnous matter. These cells are 

 irregular, rounded, or polygonal, have a simple or double 

 nucleus with nucleole (Figs. 13 and 14). Their contents, finely 

 granulous, can include accessorily granules of fat. Between these 

 cells pass very fine capillaries, emanating from the great venous 

 trunk, or vena portse, coming from the intestine (Fig. 12), 



^ Jjeydig, ffistologie Oomparie, pp. 401 — 410. — Gegenbaur, Manuel cCAtia- 

 tomie Comparie, p. 752. 



