THE LUNG PLAGUE OF CATTLE. 51 



any blood-vessels, and is not subject to the regular physiological changes 

 of a tissue, the subject of nutrition and growth, and yet a minute par- 

 ticle of this encysted, inactive mass, if inoculated on another and sus- 

 ceptible system, speedily develops the same dread disease. 



It may be justly answered that cattle bearing these encysted masses 

 in the lungs are often allowed to cohabit with healthy and susceptible 

 animals without infecting the latter. Yet we have reason to suppose 

 that the germ of lung plague does not readily live, if at all, in the 

 blood, but that when inoculated on any one part of the body, it confines 

 its ravages to that spot and those t bat are in the direct line of the lym- 

 phatic vessels leading from it. 'When inoculated on the tail it may 

 extend to the rump and pelvis, but does not show itself in the lungs. 

 Injections of the virus into the blood have in no case produced local 

 disease. (Sanderson, Duguid.) The encysted germs are only likely to 

 make their exit from the interior of the sac, by being absorbed into the 

 blood-vessels ramifying in its walls. But if the blood globulesare natu- 

 rally either unfavorable to the development of the germ, or destructive 

 to it, much more will the blood of the animal, now rendered insuscepti- 

 ble through a first attack of the disease, prove inimical to the living 

 virus. But the same acquired insusceptibility is equally true of the 

 living tissues generally of the animal bearing the encysted mass, in- 

 cluding its lymphatic vessels. It is not, therefore, to be expected that 

 the pent-up virus should readily make its escape from the encysted mass 

 so as to infect other cattle adjacent. It may, however, be conceived of 

 as escaping under one of the following conditions: 



1st. In case the insusceptibility of the subject became exhausted, as 

 happens in certain systems with every form of disease which does not 

 habitually occur a second time in the same subject. Thus we have second 

 attacks of small-pox, cow-pox, scarlet fever, measles, whooping cough, 

 and mumps, just as we have sometimes second attacks of lung plague. 

 If, therefore, in a cow bearing one of the encysted masses now under 

 consideration, the acquired insusceptibility becomes worn out, the pent- 

 up germs may suddenly find in the adjacent lung a prolific field for their 

 growth and a vantage point for a new and wide diffusion among other 

 stock. 



2d. In certain diseases, like anthrax and swine plague, a system which 

 enjoys a native or acquired immunity is still unable to resist the sud- 

 den introduction of a great excess of the disease germ or of a smaller 

 amount in a condition of unusual virulence. Thus it is that the ani- 

 mal often recovers from a small dose of the poison, but succumbs to a 

 large one; and thus, too, that in certain malignant epidemics of measles 

 or small-pox persons fall victims who have resisted exposures to a 

 milder type of the disease for a long lifetime. When therefore from any 

 cause inflammation, abscess, &c. the membrane limiting the infecting 

 mass has become broken down or unusually permeable, the insuscepti- 

 bility may be at once overcome by the great access of infecting mate- 

 rial. 



3d. In connection with the irritation of the contained mass and its 

 liquid products, the wall of the sac may become ulcerated, so as to es- 

 tablish a communication with one of the air tubes. In such a case it is 

 evident that the contents may escape and that the animal may become 

 infecting to others, though she escapes a second infection herself. 



4th. Still one other hypothesis may be hazarded. An attack of simple 

 inflammation implicating the wall of the cyst may be the occasion of 

 the escape of the poison. The cell products of inflammation are more 

 closely allied to embryonic tissue (cells) than are the elements of the 



