OF THE SECRETIONS. 101 



dowed, they select from the blood, which penetrates to their 

 remotest ramifications, the elements of the special humors 

 they are designed to elaborate. Thus the liver extracts 

 the elements of the bile ; the salivary glands the elements 

 of saliva ; the pancreas those of the pancreatic juice ; 

 and the sudoriferous glands those of the sweat, &c. 



269. Among the humors thus formed by the different 

 glands, some are immediately expelled, and the body 

 freed from them, as the sweat, the urine, &c. ; these are 

 denominated excretions. Others, on the contrary, which 

 are properly denominated secretions, are destined either to 

 be used as food for the young, as the milk ; or to take part 

 in the different functions of the body, as the saliva, the 

 tears, the gastric and pancreatic juices, and the bile. The 

 last is the most important of all the secretions, and 

 hence a liver, or some analogous organ by which bile is 

 secreted, is found in animals of every department, whilst 

 most of the other glands are only found in certain classes of 

 animals. 



270. In the vertebrates the liver is the largest of the vis- 

 cera. It is of a reddish brown color, and varies but little in 

 the different classes. In the mollusks it is no less preponde- 

 rant. In the gasteropods, like the snails, it envelops the in- 

 testine in its folds (Fig. 52) ; and in the acephala, like the 

 clam and oyster, it generally surrounds the stomach. In the 

 articulated animals it is not so compact, nor so voluminous 

 as in the mollusks. In insects it is represented by long 

 tubes variously contorted and interlaced (Fig. 51). In the 

 Eadiata this organ is largely developed, especially among 

 the echinoderms. In the star-fishes it is very large, ex- 

 tending into all the recesses of the arms ; and in color and 

 structure resembles that of the mollusks. Even in polyps 

 we find peculiar brown cells lining the stomach, which 

 probably perform functions similar to those of the liver of 

 higher animals, 



