THE FROG, A TYPICAL VERTEBRATE 



253 



As their names indicate, some of these glands manufacture 

 mucus, others poison. These substances are secreted into the 

 central cavity of the gland and then forced out upon the skin 

 through a thin tube or duct. The glands in the walls of the stom- 

 ach, which secrete gastric juice, are simple in structure, but 

 unlike the glands in the skin are sometimes branched. Other 

 glands are more complex, some resembling a bunch of grapes, like 

 the salivary glands in man. The liver and pancreas have al- 

 M. G M. g 



P.G 



will 



P. G 



Fig. 144. — A section through the skin of a frog. 

 M.G, mucous gland; P.G, poison gland. (From Holmes.) 



ready been mentioned as secreting digestive juices ; they are 

 likewise very complex. 



A few glands in both the frog and man have no ducts leading 

 to the alimentary canal or elsewhere, but their secretions pass 

 directly from their cells into the blood. These are known as 

 ductless glands and their products as internal secretions. Glands 

 with ducts, like the liver and pancreas, may also form internal 

 secretions. 



The spleen is a rounded, reddish, ductless gland that lies near 

 the end of the intestine in the frog. The function of its internal 

 secretion is not fully known, Other ductless glands are the 



