64 Causes of Disease. 



upon the mechanical obstruction to the penetration of the germ 

 into the tissues (vide pp. 18, 21). 



The interval between the entrance of the microbes into the 

 subject and the manifestation of appreciable symptoms is 

 spoken of as the period of incubation. The length of this stage 

 depends upon the vital peculiarities, the virulence and number of 

 the microorganisms, and upon the site of the infection and the 

 predisposing factors in the animal affected. If the microbes 

 are capable of rapid multiplication and of generating large 

 amounts of toxic material (as the bacteria of chicken-septicaemia, 

 which rapidly increase in the circulating blood, or the colon bac- 

 teria of mastitis which thrive luxuriantly in the milk of the 

 udder) the period of incubation is usually of but a few hours' 

 duration. Microorganisms of slow growth, as tubercle bacilli 

 and actinomycetes, induce functional disturbances only after the 

 structural changes which they bring about have attained a certain 

 grade of development, and of necessity extend their period of 

 incubation over weeks and months. Before the symptoms are 

 appreciable clinicalh' extensive anatomical changes may in many 

 cases have developed, whence it follows that a disease may be 

 latent or occult in its period of incubation, although if the animal 

 be slaughtered it is clearly seen to have been present for some 

 time. For example, in the case of pleuro-pneumonia in cattle there 

 are often found characteristic appearances of the pulmonary in- 

 flammation in animals which have been slaughtered when appar- 

 ently quite healthy ; and in hogs affected with erysipelatous valvu- 

 lar endocarditis of intense grade, the disease may be discovered 

 only on slaughtering, the animals having shown in life no 

 symptoms of a character to have suggested the existence of 

 their disease. In rabies and tetanus, in which the virus causes 

 symptoms only after it has become fixed in the cerebral nervous 

 system [the toxine in case of the latter rather than the germ 

 itself], the disease manifests itself the more rapidly the closer 

 the point of infection to the brain ; if the virus be inocu- 

 lated directly into the brain the incubation lasts but a few days, 

 while in case of ordinary subcutaneous inoculation it may be 

 prolonged to weeks, the virus being at first retained in the lymph 

 glands. Cases of infection of human beings by the bites of 

 rabid dogs manifest rabies in 8-14 days where the wounds are in 

 the face, but when the hands or feet have been bitten the period 

 of incubation lasts usually one or two months, and sometimes the 



