150 HEALTH AND DISEASE 



excessive, defective, or altered in character. An examination by means 

 of the microscope and the application of chemical tests are among the 

 means which the modern veterinarian employs to complete the evidence 

 on which his diagnosis must be based. 



It is not of course expected that the horseman will devote himself 

 to the study of symptoms sufficiently to master the subject thoroughly, 

 but he cannot fail to be interested in an account of the methods which 

 are employed by professional men to arrive at correct conclusions as to 

 the localization and pathological character of the derangements which it 

 is their object to rectify by the use of appropriate remedies. 



Special Character of Infectious Diseases. — Certain maladies 



which are due to the action of virulent micro-organisms, and also those 

 in which, up to the present time, no special microbe has been detected, 

 are distinguished from ordinary non-infectious diseases by certain features. 

 The most prominent character of all infectious maladies, which has been 

 recognized from the earliest times, is their tendency to render the system 

 of the animal which they attack proof against a recurrence of the same 

 affection for a considerable period, and in some cases for the life of the 

 subject. Further, they are marked by the occurrence of certain phases 

 or stages, beginning with what is called the period of incubation, by 

 which is meant the time which elapses from the moment of infection up 

 to the declaration of the disease, indicated by the appearance of the 

 first definite symptoms. Following this, which may be described as the 

 period of invasion, there is in eruptive affections a perfectly well-defined 

 series of changes in the character of the eruption, from its first appear- 

 ance to its decline. Small-pox furnishes the most typical instance of 

 these changes, which begin with the appearance of papules or pimples 

 (papulation). The effusion of serous fluid beneath the cuticle converts 

 the red pimples into vesicles (vesication), subsequently the contents of 

 the vesicles become purulent (pustulation), and at last dry up and form 

 a scab (desquamation), which ultimately falls off, leaving an eschar behind 

 it (pitting.) These changes occur at intervals on an average of some- 

 thing like three days. 



In infectious diseases which are not eruptive the stages are not so 

 definitely marked, but there is always the period of incubation, followed 

 by invasion indicated by febrile symptoms, then the progress to the 

 acute stage, and the gradual subsidence of the disorder to the period 

 of convalescence, or, on the other hand, the increase of the energy of 

 the attack until a fatal result occurs. 



Another peculiarity of infectious diseases is their refractory char- 

 acter in reference to remedial measures. Medicines are for the most 



