432 HUMAN BIOLOGY 



may reduce the bodily resistance; an inherited constitution 

 may render the onset of certain diseases, such as those of the 

 kidneys, heart and blood vessels, or lungs, a definite danger; 

 and one disease may favor the onset of another, for example 

 measles and diabetes melhtus may both be followed by 

 tuberculosis (see Chap. xx). Realization of thtse various 

 tendencies enables precautionary steps to be taken. Epidem- 

 iology, or the study of the conditions responsible for the 

 great epidemics, such as bubonic plague and cholera, has 

 provided knowledge bearing on the prevention and control 

 of their occurrence, as is shown by their practical dis- 

 appearance from civilized countries where preventive meas- 

 ures are adopted, and by their occurrence in India and China. 

 Investigation of the factors favoring the onset of chronic 

 disabling disease, such as was advocated and begun by the 

 late James Mackenzie, still awaits completion. In order to 

 understand and remove the disposing factors an exhaustive 

 study must be made not only of the earliest symptoms and 

 signs of disease, but of the patient's environment; circum- 

 stances in the patient's life which are unfavorable to a 

 healthy existence may be obvious to the medical man, who 

 may thus be able to benefit other members of the family. 

 ' The earliest indications of disease, such as undue fatigue 

 or a sense of ill-being, are manifestations of disordered 

 function, and often precede any gross structural change 

 which can be detected by ordinary physical examination 

 of the patient. In the past the decision whether or not there 

 was anything the matter with a person largely turned on the 

 presence or absence of gross physical changes. The study of 

 functional efficiency of different organs of the body has 

 much advanced the recognition and treatment of disease; 

 chemical examination of the contents of the stomach after a 

 test-meal may show absence of the hydrochloric acid nor- 

 mally present and necessary for the digestion of proteins 

 (meaty foods) and this deficiency should be corrected by 

 acid given by the mouth. Chemical analyses of the blood, 

 the air expired from the lungs, the urine and the excreta 

 provide evidence of the manner in which the changes in 

 the living body producing heat and energy (metabolism) 

 are being carried out; examination of the blood will show 



