INFLUENCE OF NERVOUS SYSTEM. 389 



vation of Magendie and Mayer, that inflammation of the 

 eye is a constant result of ligature of the sympathetic 

 nerve in the neck, and many other observations of a 

 similar kind, exhibit very well the influence of the latter 

 nerve in nutrition. 



4. The fourth condition necessary to healthy nutrition is 

 a healthy state of the part to be nourished. This seems 

 proved by the very nature of the process, which consists in 

 the formation of new parts like those already existing; for, 

 unless the latter are healthy, the former cannot be so. 

 Whatever be the condition of a part, it is apt to be per- 

 petuated by assimilating exactly to itself, and endowing 

 with all its peculiarities the new particles which it forms 

 to replace those that degenerate. So long as a part is 

 healthy, and the other conditions of healthy nutrition exist, 

 it maintains its healthy condition. But, according to the 

 same law, if the structure of a part be diseased or in any 

 way altered from its natural condition, the alteration is 

 maintained ; the altered, like the healthy structure, is per- 

 petuated. 



The same exactness of the assimilation of the new parts 

 to the old, which is seen in the nutrition of the healthy 

 tissues, may be observed also in those that are formed in 

 disease. By it, the exact form and relative size of a 

 cicatrix are preserved from year to year ; by it, the thick- 

 ening and induration to which imflammation gives rise are 

 kept up, and the various morbid states of the blood in 

 struma, syphilis, and other chronic diseases are maintained, 

 notwithstanding all diversities of diet. By this precision 

 of the assimilating process, may be explained the law that 

 certain diseases occur only once in the same person, and 

 that certain others are apt to recur frequently ; because in 

 both cases alike, the alteration produced by the first attack 

 of the disease is maintained by the exact likeness which the 

 new parts bear to the old ones. 



The period, however, during which an alteration of 



