46 SECRETION 



2. That the spleen destroys the red blood corpuscles. The 

 evidence for this theory is that spleen tissue is rich in iron- 

 holding compounds, and that certain ameboid cells of the 

 spleen have been seen apparently ingesting and destroying 

 red blood cells. 



3. That the spleen produces uric acid. Uric acid is found 

 in the spleen, but also in all lymphoid tissue. 



4. That the spleen produces an enzyme which is carried to 

 the pancreas in the blood, converting trypsinogen into trypsin. 

 A striking feature of the spleen is its rhythmic movements. It 

 undergoes a slow expansion and relaxation, with definite periods 

 of digestion. These are due to vasomotor changes, the maxi- 

 mum vasodilatation occurring about the fifth hour after a 

 meal. In cats and dogs there are, in addition, rhythmical 

 changes taking place from minute to minute which serve to 

 maintain a constant circulation through the organ. The spleen 

 is well supplied with nerves, stimulation of which produce' a 

 contraction. 



The chemical substances found in the spleen are interesting, 

 since they indicate a marked metabolism. There is a large 

 percentage of iron in an unknown organic combination. In 

 addition, there are fatty acids, fats, cholesterin, xanthin, 

 hypoxanthin, adenin, guanin, and uric acid. 



Thymus. There seems to exist a reciprocal influence 

 between the thymus and the sexual glands. The thymus 

 persists longer than normal in castrated animals, and premature 

 removal of the thymus is followed by a rapid growth of the 

 testes. In young mammals the removal of the thymus causes 

 a transient disturbance in nutrition; a temporary decrease 

 in the number of leukocytes and a diminished resistance to 

 pus-forming organisms. 



Ovary and Testis. In addition to the prime function 

 of the sexual organs as organs of reproduction there is much 

 evidence that they influence the processes of growth, partly, 

 at least, through the production of an internal secretion. Dur- 

 ing the breeding season the muscles of the forearm of Rana 

 fusca become hypertrophied in order to hold the female more 

 firmly. At the same time the balls of the toes increase in size 

 and become covered with a black growth. In castrated frogs 

 these events do not occur, but they return if a piece of testicle 



