XII INTERNAL SECRETION AND DUCTLESS GLANDS 221 



The Spleen. — The spkeji of the frog is a rounded, reddish 

 body lying dorsal to the anterior end of the cloaca, where it 

 is attached to the supporting mesentery. It receives blood 

 from a branch of the anterior mesenteric artery, and gives 

 off the splenic vein, which forms a branch of the hepatic 

 portal system ; both blood vessels enter at a common point 

 called the hilus. The spleen is surrounded by a fibrous 

 membrane outside of which the greater part of the surface 

 is coated with peritoneum. The inner framework of the 

 spleen consists of a network of areolar tissue which contains 

 the essential part of the organ, the spleen pulp. The latter 

 is composed of several kinds of cells, many of which repre- 

 sent stages in the development of leucocytes, of which the 

 spleen contains a large number. There are numbers of 

 large cells containing an abundance of pigment, both 

 yellow and black. The pigmented cells have the property 

 of absorbing pigment granules with which they come in 

 contact ; if coloring matters are injected into the blood, they 

 are taken up by these cells in large quantities (Ponfick, 

 Siebel). The spleen also contains large cells in which red 

 blood corpuscles are frequently found in all stages of degen- 

 eration. 



The spleen is an organ having various functions. It is a 

 place where red blood corpuscles are destroyed, probably 

 when they have reached a moribund condition. Pigment 

 and other foreign matters in the blood are taken up by 

 certain cells of the pulp. Leucocytes are in all probability 

 formed in the spleen, as various stages in their production 

 have been observed, and it has been found that there is a 

 greater number of these cells in the blood of the splenic vein 

 than in that of the splenic artery. 



According to some investigators the spleen produces an 

 internal secretion which acts upon the pancreas so as to 



