124 DISEASE IN WILD MAMMALS AND BIRDS 
tumors than do the mammals. The mononuclears of avian 
blood rise normally as high as 60 per cent., of which 
40-45 per cent, are of the small size. 
The size of the spleen is subject to great variation 
not only within orders but actually within genera ; it even 
seems that one sometimes sees two or three members of 
a species kept in the same enclosure, maybe with the same 
disease, maybe without any obvious disease, yet with 
definite variation in the size of the organ. This irregu- 
larity is recognized by veterinarians (Hutyra and Marek) 
who ascribe it to some unkno^vn disease, past or present, 
and to normal variation. The lack of uniformity is seen 
more clearly in birds than mammals. In the former it 
might be due at times to low grade or inactive parasitism 
or, conjecturally, to toxins from incorrect diet; we shall 
see later that infection and intoxication seem to have a 
different effect. Some writers have mentioned the possi- 
bility of an idiopathic splenomegaly (?), a condition 
associated in youthful human beings, with anemia and 
lymphadenopathy, and occasionally going over into a 
sort of leucemia. The existence of such a condition is 
difficult to admit or deny; we have met nothing which 
could not be aligned in some fairly well defined group. 
Birds have a relatively larger spleen than mammals and 
in addition the organ seems to respond more actively 
in infections or intoxications, since it may reach, under 
active stimulation, a siz;e ten times that of the nor- 
mal organ. 
Enlargements of Spleen. 
Acute enlargements of the spleen, be they of con- 
gestive or ''inflammatory" nature are quite common 
among the lower animals, a fact that is recognized in 
veterinary medicine. Congestions of the spleen are most 
often seen during acute infections and diseases of the 
heart and liver. The diagnosis has been made among 
