250 THE LYMPHATIC SYSTEM 



proliferation and the ingestion of erythroplastid debris by the splenic 

 and endothelial cells, and by the fact that the proportion of lymphocytes 

 to erythroplastids in the splenic vein is very much greater than in the 

 splenic artery. After severe hemorrhage or certain anemias the spleen 

 may resume its fetal erythropoietic function. Numerous small super- 

 numerary spleens, of varying size but usually about the size of a pea, 

 are frequently found in the vicinity of the spleen. 



Besides the production of lymphocytes and the destruction of senile 

 red corpuscles, the normal adult spleen has been credited also with an eryth- 

 ropoietic role, and with a function concerned with the metabolism of iron. 

 That the spleen does not, however, have any specific function absolutely 

 essential to life is proved by the fact that it may be removed without 

 serious consequence. Obviously its function may be taken over by some 

 other organs. Such compensatory role is usually attributed to the hemo- 

 lymph nodes and the red marrow. However, splenectomy in dogs is not 

 followed by increase in the number or size of the hemolymph nodes, nor by 

 a production of accessory spleens (Meyer, Jour. Exp. Zool., 16, 2, 1914). 

 Removal of the spleen in dogs seems to exert a stimulating effect upon the 

 formation of red cells in bone-marrow (Krumbhaar and Musser, Jour. Exp. 

 Med., 20, 2, 1914). Pearse and Pepper (Jour. Exp. Med., 20, 1, 1914) found 

 that splenectomy caused a transformation of yellow into red marrow. The 

 result is interpreted as showing that in the absence of the spleen the mar- 

 row may take on the function of storing and elaborating the iron of the 

 blood pigment for future utilization by new red cells. The experiments 

 of Austin and Pearse (Jour. Exp. Med., 20, 2, 1914) on the contrary lead 

 them to conclude that the spleen does not exert a constant and important 

 function on iron metabolism. The complete function of the spleen appar- 

 ently remains largely unknown. Like the thymus it is sometimes classified 

 among the organs of internal secretion. 



Development. The anlage of the spleen appears at the beginning of 

 the second month as a condensation and swelling in the mesenchyma on 

 the left border of the dorsal mesogastrium. The overlying mesothelium 

 proliferates extensively and its cells invade the mesenchyma obliterating 

 the line of demarcation. The early histogenesis is obscure. The mesen- 

 chyma is potentially capable of producing all the definitive elements of the 

 spleen: connective tissue capsule and framework, and lymphocytes. The 

 mesothelium, genetically closely related to mesenchyma, probably aids in 

 the general process. Probably also the bulk of the later lymphocytes invade 

 the spleen from without. 



