CIRCULATION AND RESPIRATION 143 



Spleen. — The spleen occurs as a distinct organ in all of the 

 vertebrates except the Cyclostomata. In the frog it develops in 

 the mesentery of the intestine; in man in the mesentery of the 

 stomach. It has a rich blood supply but no lymphatic vessels, 

 in which it resembles a hemolymph gland. The capillary cir- 

 culation is peculiar in that in addition to the usual capillaries 

 connecting arteries and veins there is also a series of venous 

 sinuses in parallel with the capillaries. Thus arterial blood may 

 reach the veins either through the capillaries or through the 

 sinuses. The sinuses therefore serve as a by-pass to the capil- 

 laries, in which blood may accumulate in large quantities under 

 certain conditions and be diverted from the general circulation 

 (Fig. 90). In man the cells of the splenic tissue, known as the 

 spleen pulp, consist of erythrocytes, lymphocytes, and phago- 

 cytic splenic cells resembling leucocytes. 



The precise function of the spleen is unknown. It has long 

 been thought to have to do with the normal destruction of red 

 corpuscles, but agreement on this point is lacking. After a meal 

 the spleen enlarges, reaches a maximum in five hours and then 

 slowly contracts. The presence of venous sinuses accounts for 

 the changes in volume, since these are brought about by the 

 storage and release of blood from the sinuses. 



