310 NUTRITION. 



the nutrition of the tissues and organs to which they are sup- 

 plied. Such special nerves are called trophic nerves (see chap- 

 ter on the Nervous System). 



It is not at present possible to say whether the influence on 

 nutrition is exercised through the cerebro-spinal or through the 

 sympathetic nerves, which, in the parts on which the observa- 

 tion has been made, are generally combined in the same sheath. 

 The truth perhaps is, that it may be exerted through either or 

 both of these nerves. The defect of nutrition which ensues 

 after lesion of the spinal cord alone, the sympathetic nerves 

 being uninjured, and the general atrophy which sometimes 

 occurs in consequence of diseases of the brain, seem to prove 

 the influence of the cerebro-spinal system : while the observa- 

 tion 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 forma- 

 tion 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 perpetuated 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, ac- 

 cording 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 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 per- 

 son, 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 structure 



