246 STRUCTURE OF GLANDS. 



418. Of the secretions thus formed by the different glands, 

 ' some are immediately expelled from the body, as the sweat, 

 the urine, &c. ; these are denominated excretions. Others, 

 on the contrary, 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, which are properly denominated secretions. 

 Of all the secretions, if we except that from the lungs, the bile is 

 the most important; and hence a liver, or some analogous organ 

 by which bile is secreted, is found in all animals, while some or 

 all of the other glands are wanting in the lower classes. In the 

 vertebrata the liver is the largest of all the organs of the body. 

 In the mollusca it is no less preponderant. In the gastero- 

 poda, like the snails, it envelops the intestine in its convolu- 

 tions (fig. 177) ; and in the conchifera, like the clam and oyster 

 (fig. 176), it generally surrounds the stomach. In insects it 

 is in the form of long tubes variously contorted and interlaced 

 (fig. 179). In the radiata this organ is largely developed, 

 especially among the echinoderms. In the star-fishes (fig. 36) 

 it extends into all the recesses of the rays ; and in colour and 

 structure resembles the liver of the mollusca. Even in bryo- 

 zoan polyps (fig. 1 75) we find brown cells lining the digestive 

 cavity, which probably perform functions similar to those of 

 the liver of higher animals. 



STRUCTURE OF GLANDS. 



[ 419. The type or elementary form of every secreting 

 gland is either a simple capsule, an elongated blind sac, or a 

 rounded vesicle, upon the outer aspect of which vessels are 

 ramified, and which on the inside generally exhibits numbers 

 of small cellular projections or depressions, and an outlet 

 through which the secreted matter escapes. Many of the 

 cutaneous and mucous glands, as also the simple glands of 

 the stomachs of birds (fig. 186, B. , d), and the Lieberkiih- 

 nian glands of the intestines, afford examples in point ; but 

 they soon begin to get more complex, coalescing, dividing, 

 and sending forth new lateral lobules (fig. 185, B. e), and 

 by repetitions of the same process even acquiring a pretty 

 complicated mulberry appearance (fig. 184, B. /). The 

 ventricular glands of mammals are already somewhat more 

 compound (fig. 181, et seq.). The extent of secreting sur- 



