THE NERVOUS SYSTEM. 157 



irritation lasts. Further evidence of the influence of the nervous system 

 upon nutrition is furnished by those cases ir which, from mental anguish, 

 or in severe neuralgic headaches, the hali' becomes grey very quickly, or 

 even in a few hours. 



So many and varied facts leave little doubt that the nervous system 

 exercises an influence over nutrition as over other organic processes; and 

 they cannot be easily explained by supposing that the changes in the 

 nutritive processes are only due to the variations in the size of the blood- 

 vessels supplying the affected parts, although this is, doubtless, one im- 

 portant element in producing the result. 



The question remains, through what class of nerves is the influence 

 exerted? When defective nutrition occurs in parts rendered inactive by 

 injury of the motor nerve alone, as in the muscles and other tissues of a 

 paralyzed face or limb, it may appear as if the atrophy were the direct 

 consequence of the loss of power in the motor nerves; but it is more prob- 

 able that the atrophy is the consequence of the want of exercise of the 

 parts; for if the muscles be exercised by artificial irritation of their nerves 

 their nutrition will be less defective. The defect of the nutritive process 

 which ensues in the face and other parts, however, in consequence of 

 destruction of the trigeminal nerve, cannot be referred to loss of influence 

 of any motor nerves; for the motor-nerves of the face and eye, as well as 

 the olfactory and optic, have no share in the defective nutrition which 

 follows injury of the trigeminal nerve; and one or all of them may be 

 destroyed without any direct disturbance of the nutrition of the parts 

 they severally supply. 



It must be concluded, therefore, that the influence which is exercised 

 by nerves over the nutrition of parts to which they are distributed is to 

 be referred, in part or altogether, either to the nerves of common sensa- 

 tion, or to the vaso-motor nerves, or, as it is by some supposed, to nerve 

 fibres (trophic nerves), which preside specially over the nutrition of the 

 tissues and organs to which they are supplied. 



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 observation has been made, are generally 

 combined in the same sheath. The truth perhaps is, that it may be ex- 

 erted 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 con- 

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

 cerebro-spinal system: while the observation 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. 



