CONTAGION AND CONTAGIOUS DISEASES. 133 



some of them excite fermentation, others putrefaction. The 

 fermentative ones have the power of peptonising and liquefying 

 culture media containing gelatine, and in many instances of 

 producing a poisonous alkaloid — ptomaine — which causes septic 

 poisoning, a condition distinct from septic infection. The effect 

 of septic poisoning depends upon the dose, a small dose produc- 

 ing only mild and transient effects, whilst a large one may induce 

 a fatal result ; whereas in septic infection a small dose may in- 

 duce severe disease and death, because the poison introduced is 

 a living organism capable of propagation and numerical increase. 



The researches of Brierger have enabled him to isolate various 

 ptomaines from dead bodies, putrid meat, fish, and cheese — 

 cadaverin, putrescin, saprin, pcptotoxin, and many others, vary- 

 ing in their toxic properties. 



Associated with or independent of the production of the 

 poisonous alkaloids, a poisonous albumose is secreted in culture 

 media, and in addition the following substances are evolved : — 



(a.) Gases. — Carbonic acid, hydrogen, light carburetted hydro- 

 gen, sulphuretted hydrogen, and ammonia. 



(&.) Nitrates, water, and sulphur. 



(c.) Volatile substances (trimethylamin, alcohol, formic acid, 

 acetic acid, propionic acid, butyric acid. 



[d^ Fixed acids, — lactic, malic, succinic, oxalic, and tartaric. 



(e.) Taurin, leucin, alanin. 



(/.) Tyrosin, phenol and kresol (aromatic bodies). 



(_^.) Carbo-hydrates, albumoses, peptone, hydrolytic ferments. 



(A.) Colouring matters. 



When germs gain entrance into the animal economy, a 

 kind of battle royal takes place between them and a defen- 

 sive army of phagocytes, consisting mostly of white corpuscles, 

 aided by the connective tissue corpuscles, the endothelial 

 cells of the capillary vessels, soft epithelial and the cells 

 of muscular fibres. These endeavour to arrest the invasion of 

 the microbes by attempting to destroy them. When they fail in 

 this the disease is developed, when successful in their efforts 

 the disease is not developed ; when the virulent germs are 

 numerous and strong, they, in a susceptible subject, paralyse 

 and overcome the phagocytes ; but wdien the virus is attenu- 

 ated, or when the subject is non-susceptible or immune, the 



