In tuberculosis of this organ the capsule is almost invariably thickened over the nodules, but not over the whole organ and rarely in the diffuse variety. There is almost always, however, in this prolonged infectious disease some reaction on the part of the spleen, particularly when several other viscera are involved. In such cases a low grade of general hyperplasia and fibrous tissue increase is found. This is especially true in the Primates and Carnivora in long continued pulmonary tuberculosis, and it is in this form that one does not often see tuberculous lesions proper in the spleen itself. Tuberculosis of the spleen is not very common in adult human beings and domesticated animals, while in the young it is seen in a fairly high percentage of cases. In the wild animals this organ seems much more susceptible to the settling of tubercle bacilli as will be attested by the facts to be cited, and yet there are some conspicuous exceptions to this statement. Just why certain groups, or orders should show splenic tuberculosis very frequently while others fail to do so is far from clear. The appearance of the lesion is fairly comparable throughout the animal kingdom, variations among the Aves being chiefly in the more frequent occurrence of the diffuse tuberculous splenitis. Miliary, caseous and conglomerate masses occur separately or together and without very distinct relation to other pathological involvements. The relative incidence of tuberculosis in the spleen is set forth in the section devoted to this infection, but may be mentioned here in a general way. The organ is about equally susceptible to the disease in mammals and birds according to the figures, but the high percentage for the former is due to the frequency with which tuberculosis is met in the Primates. With this order deducted there is no doubt that the avian spleen has a greater vulnerability for the tubercle bacillus than has the mammalian organ. Rodentia as a mammalian order stand next to the Primates, whereas the Carnivora and Ungulata relatively seldom show splenic tuberculosis. Among the Aves one finds that Columbæ, Galli and Accipitres have the highest splenic susceptibility, but beyond this one hardly dare venture because of the irregularity in the number of specimens seen at autopsy. Suffice it to say that a greater number of cases of tuberculosis are seen in the spleen of the class for which the intestinal origin of tuberculosis appears most important.