INTRODUCTORY. 245 



the minute changes which it undergoes in disease. The cellu- 

 lar theory has done for pathology what the atomic theory has 

 for chemistry. 



Medical knowledge has been advanced more in recent 

 years by experimental pathology than by any other method of 

 investigation. Though Galen is said to have used living 

 animals for pathological experimentation, the foundation of 

 these methods and the revival of pathology in general were 

 due to John Hunter in the latter part of the last century. To 

 bacteriology, one branch of experimental pathology, we owe 

 our knowledge of the microbic cause of many of the infec- 

 tious diseases the germ-theory, in its practical results having 

 almost revolutionized medicine and surgery within a quarter 

 of a century. 



Disease is a deterioration in or deviation from the normal 

 standard called health. Probably all deviations of function 

 from normal are dependent on some alteration in structure ; 

 when our present methods fail to discover such anatomical 

 changes the disease is said to be functional, in contradistinction 

 to organic disease, in which such alterations can be demon- 

 strated. There is no sharp line of demarcation between 

 health and disease, the one passing imperceptibly into the 

 other. 



Heredity plays a most important part in the causation of 

 disease ; it is not uncommon to see reproduced in a family the 

 same infirmities or diatheses for many generations and the ill 

 results of consanguineous marriage are a matter of common 

 knowledge. 



Susceptibility to various diseases varies greatly in different 

 individuals the determining factors being inherited in some 

 cases and acquired in others, and temporary or permanent in 

 their duration. The susceptibility of children of tubercular 

 parents to tuberculosis is observed with great frequency ; 

 without such inherited tendencies however, during some tem- 

 porary impairment of health, there may be an acquired 

 susceptibility to the disease, which, for instance, is especially 

 common after measles in children the attack either preparing 

 a soil particularly inviting to the tubercle bacilli or reducing 

 the patient's resisting powers to such an extent that they are 

 not able to cope with the invasion of these micro-organisms 



