ETIOLOGY— CAUSES OF DISEASE 217 



Excessive cold produces debility by its immediate sedative influence 

 on the system, and the repellent effect of cold upon the surface diminishes 

 the circulation in the superficial vessels of the skin and leads to an accumu- 

 lation of blood in some of the internal structures, acting particularly upon 

 some parts of the digestive and circulatory system. Its influence upon 

 the nerve centres, whether it is direct or indirect, is shown in the rapid 

 diminution of the vital powers of resistance, a condition which is eminently 

 favourable to the action of exciting causes of disease. 



Heredity. — Of the existence of a tendency to disease transmitted 

 from parents to offspring no doubt can be entertained; whether the pre- 

 disposition so conveyed be called idiosyncrasy, constitutional peculiarity, 

 remarkable liability, or special receptivity, or be designated by any other 

 form of words, the fact remains that the influence of heredity has always 

 been recognized, although the term itself may not have been used by the 

 older writers. It may be defined as the law of inheritance not neces- 

 sarily limited to the transmission of a tendency to disease, but applying 

 equally to everything which belongs to the living organism, every quality, 

 indeed, whether good or bad, whether relating to mental functions 

 or to organic structure. A modification in the character of organs due 

 to the manner of their employment in a former generation has often been 

 observed; thus large hands are inherited by people whose ancestors led 

 laborious lives, while small hands belong to the descendants of those who 

 have not been compelled to follow occupations which require manual 

 work. In the course of the observations made by Darwin many instances 

 are mentioned of the appearance of peculiarity of structure and of certain 

 diseases at particular periods of life in certain families, and among the 

 cases quoted are blindness occurring in a family and being transmitted to 

 twenty-seven children and grandchildren, the affection beginning always 

 between the fifteenth and sixteenth year, and ending in a total loss of 

 sight at the age of twenty-two. Also of insanity in a family affecting 

 grandfather, father, and son, who all became insane and committed suicide 

 at the age of fifty; in fact, the evidence offered by biologists in favour 

 of the remarkable influence of heredity in continuing a tendency to cer- 

 tain diseases is so complete that it is impossible to question the fact. It 

 is indeed perfectly well known that scrofula, asthma, rheumatism, gout, 

 epilepsy, blindness, and deafness run in families— not necessarily in one 

 continuous line, but commonly interrupted, even a whole generation being 

 passed over, the disease appearing in the next. 



Temperament. — In dealing with the predisposing causes of disease, 

 certain types of temperament are defined. The sanguine temperament 

 refers to animals of a lively disposition with active circulation and quick 



