388 NUTRITION. 



as in the muscles and other tissues of a paralysed face or 

 limb, it may appear as if the atrophy were the direct con- 

 sequence of the loss of power in the motor nerves ; but it 

 is more probable 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 nutri- 

 tion will be less defective (J. Reid). The defect of the nutri- 

 tive process which ensues in the face and other parts, 

 moreover, 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 nutri- 

 tion which follows injury of the trigeminal nerve; and one 

 or all of them may be destroyed without any direct disturb- 

 ance 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 either to those among 

 their branches which conduct impressions to the brain and 

 spinal cord, namely, the nerves of common sensation, or, 

 as it is by some supposed, by nerve-fibres, which preside 

 specially over the nutrition of the tissues and organs 

 to which they are supplied. Such special nerves are 

 called trophic nerves (see Chapter 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 observation has been made, are generally com- 

 bined 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 con- 

 sequence of diseases of the brain, seem to prove the 

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



