DISEASES OF THE LYMPHATIC TISSUES 123 
is relatively smaller than birds of somewhat comparable 
size (the ostrich, for example). The Carnivora and 
Rodentia among the mammals, have quite large spleens 
proportionately, while comparable birds, Aceipitres and 
Striges, have relatively small ones, and an absolutely 
small amount of marrow. 
Another anomalous example of the function of this 
tissue is found in the hematopoiesis of birds. While there 
seems to be some evidence that in the spleen and liver 
red blood cells may be formed, there is perhaps at most 
times sufficient bone marrow to keep the blood cells at a 
proper number. There is no inverse relation between the 
amount of bone marrow and that of splenic bulk ; that is 
in diving birds with their hollow bones the latter is no 
greater than in gallinaceous birds which have abundant 
marrow in all the skeleton. 
It has been suggested that the spleen supplies an 
activator for pancreatic ferments. This receives a sort of 
support from the greater size of the organ in carnivores 
and smaller relative size in ungulates, since in the former 
concentrations of digestive enzymes are more often 
needed. Because of the greater excretion of iron in 
splenectomized animals or those being starved, Fischer 
has thought that the spleen has some power to metabolize 
this element ; this receives some support from the state- 
ments concerning the size of this organ in the carnivorous 
orders. From experimental studies it appears that some 
relation exists between the lymphocytes and resistance to 
implanted tumors. If one apply this idea to the amount 
of lymphatic tissue and the incidence of spontaneous new 
growths, it is found that no definite relationship is discov- 
erable either in terms of size of spleen or richness in 
lymphatic nodes. Carnivores, rodents and marsupials 
show a high tumor incidence and have a good lymphatic 
supply. Aves, on the other hand, with a poor nodal appa- 
ratus and a variable splenic bulk, show many fewer 
