110 EEPOET OF THE BUREAU OF ANIMAL INDUSTRY. 



the lungs. The course of the invasion shows that the virus is not 

 inhaled with the air, that it is not suspended in the air as a living 

 germ, otherwise it would "be difficult to explain the peculiar localiza- 

 tion and the slowly progressive nature of the disease. A virus im- 

 bedded in some liquid vehicle will perhaps explain all the facts most 

 satisfactorily. Its slow movement from bronchus to bronchus, its 

 limitation by gravity to the most dependent portions of the lungs at 

 first, and its extension upwards as it gains upon the vitality of the 

 lung tissue, all accord with this theory. At the same time it har- 

 monizes with the fact that the microbe causing the disease can not 

 survive drying for even a single day. 



In this disease the lungs are primarily the seat of the virus, and in 

 them the greatest changes are observable. The lesions are those of 

 a broncho-pneumonia. The pleura is secondarily involved over the 

 seat of the disease when this extends to the surface of the lungs. 

 The great variety in the appearance and extent of the lesions as 

 manifested in different cases may be brought together under a few 

 heads for description. 



The most severe types of disease are encountered at the beginning 

 of an epidemic, and may be conveniently denominated acute. Plates 

 I and II are illustrations made from the right lung of pig No. 407, 

 described in the preceding pages. As may be seen, there is exten- 

 sive pleurisy accompanying the pneumonia. The disease is charac- 

 terized by a solidification of the ventral, cephalic, and median lobes, 

 and a portion of the principal lobe, usually of both lungs. The dis- 

 eased lobes are moderately expanded, so that the thorax seems almost 

 filled up with lung tissue when the sternum is removed. The hepa- 

 tized portion has a bright blood-red color, when viewed from the 

 surface, as well as on section. The surface in many cases has a 

 peculiar mottled aspect, shown in Plate III, fig. 2. The bright-red 

 ground is dotted with closely set, grayish-yellow points, arranged 

 quite regularly in groups of four, occasionally of three. These points 

 are not sharply defined, but hazy. When examined with a lens this 

 haziness is well marked. This grayish mottling does not appear 

 everywhere on the diseased lung, but only upon some lobes, and then 

 with striking clearness and uniformity. These points no doubt are 

 the terminal air sacs, or infundibula distended with the cellular ex- 

 udate. The more leucocytes in the exudate the whiter the injection 

 will appear through the translucent pleura. The bacteria are found 

 imbedded in the cellular masses, which occlude the alveoli. The dis- 

 ease involves the terminal air tubes, as they are frequently found 

 packed with cells. The larger bronchioles and bronchi are the seat 

 of catarrhal changes. The lumen of the tubes is filled with a muco- 

 purulent secretion,' usually containing large numbers of bacteria. 



The foregoing may be regarded as the early stages of the disease 

 proper. When the invasion is thus extensive and takes place sud- 

 denly, the animal speedily succumbs before the disease has had the 

 opportunity of entering upon the more advanced stages. But in per- 

 haps the majority of animals the disease progresses very slowly. 

 It may be that only the ventral lobes are attacked at first, and then 

 only in certain limited areas. The surrounding tissue becomes hyper- 

 semic and often consolidated. The areas first attacked become con- 

 verted into homogeneous greenish or yellowish white masses, sharply 

 defined from the surrounding tissue; They cut like ordinary hard 

 cheese, and on microscopic examination are found to be made up of 

 dead lymphoid cells and bacteria of all kinds. The process of case- 



