394 NUTRITION. 



general atrophy which sometimes occurs in consequence 

 of diseases of the brain, seem to prove the influence of the 

 cerebro-spinal system : while the observation of Magdndie 

 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 

 perpetuated. 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 

 thickening and induration to which inflammation 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 like- 

 ness which the new parts bear to the old ones. 



