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 irregularity is recognized by veterinarians (Hutyra and Marek) who ascribe it to some unknown 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 possibility 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 size ten times that of the normal organ. ENLARGEMENTS OF SPLEEN. Acute enlargements of the spleen, be they of congestive 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 mammals proportionately more than in birds, 27 or 1.5 per cent. to 35 to 1 per cent. REASON FOR CONGESTIVE ENLARGEMENTS. Perhaps this is partly due to the circulatory anatomy since in the former the arterial and venous supply is usually by one large vessel of each kind, whereas in birds the splenic branch of the cœliac axis breaks up into several small arteries and the venous return is accomplished by numerous venules some of which reënter the posterior cava almost directly, others joining with the mesenteric to form the portal; by this arrangement a more elastic system is assured. The same condition is found when analyzing the records of chronic passive congestion; in eighteen recorded cases, fourteen were mammals and four birds. In these cases the principal associated pathological lesions were pleural and pulmonary in seven, cardiac in three, renal in three and hepatic in four. While the anatomy of the splenic blood supply may help to explain the small number of congestions in Aves, it will not answer for the inflammations which occur in large numbers in this class; this will be discussed in the following pages.