Pathogenesis. 297 



localization by transmission with the blood stream. In these 

 parts the virus propagates, and causes the development of the 

 characteristic exanthema of pox. The epithelial cells in the 

 stratum mucosum swell, changing to pale flakes without nucleus, 

 and then dissolve completely in the serous fluid. Some of these 

 cells resist this process, and are under the pressure of the 

 accumulating exudate drawn out to fine threads which then form 

 a loose network in the inside of the vesicle. At this stage 

 the contents of the vesicle are enclosed, on the inside by the 

 papillary body, w^liich may be still covered by a thin layer of 

 epithelium, or may be entirely denuded, and at the same time 

 is infiltrated with small round cells. From the outside it is 

 closed by the corneal layer. Later a navel-like depression (pit) 

 forms on the top of the vesicle, the development of which 

 is explained in various ways. According to Rindfieisch the 

 depression in the covering of the vesicle is caused by the horny 

 layer of the epidermis of the hair follicle, or the excretory 

 duct of the sweat gland around which the pox is arranged 

 in a concentric manner. According to the view of Virchow and 

 Weigert the crater-like depression is formed because the 

 central necrotic part of the pox, which does not take part 

 in the expansion, acts as a retinaculum of the covering of the 

 vesicle, while the progressive accumulation of pus extends the 

 periphery of the pox. As a result of the migration of leucocytes 

 the contents of the vessels become purulent, and at the same 

 time the network in the inside undergoes dissolution. Finally 

 a scab forms, owing to the drying of the contents under which 

 the corium becomes covered by fresh epithelium; but if the 

 inflammation extended to the deeper parts, a scarred depression 

 remains at the place of the destroyed papilla. 



This pecuhar pathological process develops to maturity 

 as the result of the pathogenic action of the pox virus itself, 

 while the subsequent suppuration of the contents of the vesicles 

 is produced by pyogenic bacteria, which probably penetrate 

 from the glandular duct and the hair follicles of the skin, into 

 the inside of the vesicles. According to their nature and viru- 

 lence the inflammatory process may be more or less intensely 

 influenced by them. Especially the occasional hemorrhages 

 and the deeper destruction of the corium, as well as the subse- 

 quent pyemia or septicemia should be considered only as ac- 

 cidental infections, and therefore the mild or severe character 

 of different outbreaks also depends on such secondary infec- 

 tion. At places which are protected from this the virus of 

 pox propagates without foreign contamination, and in sucli 

 cases, as for instance in the hard nodules wliicli sometimes 

 occur under the skin of sheep, the contents consist of virulent 

 pox hanph which is free of pyogenic bacteria. 



Whether the eruptions of pox remain local or become 

 generalized depends, aside from the susceptibility of the re- 

 spective animal species, upon the virulence of the virus and 



