CHAP, ix.] NEEVOUS SYSTEM AND ORGANS OF SENSE. 309 



31. The so-called FUNCTIONS of particular nerves are then partly 

 learned through their distribution, partly through experiments, and 

 partly by the very simplest observations. Those of the CRANIAL 

 NERVES, for instance, which minister to special sense, are plainly 

 and obviously distinguishable from all others. Mechanical 

 stimulation of the optic or auditory nerves does not produce pain, 

 but only certain sensations either of light or noise. Division of 

 the various cranial nerves causes paralysis or insensibility, or both, 

 to the parts they supply, in accordance with their distribution and 

 what has already been said as to their several functions. 



Thus, division of the seventh nerve causes distortion of the mouth 

 from paralysis of the muscles of the face, except those supplied by 

 the fifth nerve. 



The integrity of the pneumogastric nerve is needed for the experi- 

 ence of the sensations of oppression from want of air, of irritation in 

 the air passages, of hunger, thirst, &c., as also for the due performance 

 of the functions of the parts supplied by it. Thus its division paralyses 

 the movements of the stomach (so that food is only digested at its 

 surface,) and diminishes its secreting power, as well as that of the 

 liver, and renders deglutition impossible finally even producing 

 suffocation. Yet it accelerates the action of the heart, which is, on 

 the contrary, impeded or stopped by irritation of the pneumogastric, 

 so that it seems that this nerve acts normally as a check on the 

 heart's action. Division of the spinal accessory nerves paralyses 

 the action of swallowing and also of the laryngeal apparatus and so 

 destroys the voice, though the respiration action (being under the 

 influence of the pneumogastric) continues. Division of the hypo- 

 glossal nerves, of course, paralyses the tongue. A short summary 

 of the functions of all the cranial nerves has been already * given. 



32. The FUNCTION OF THE SPINAL CORD is commonly considered 

 to be merely a conductor of both sensitive influence to the brain and 

 of motor (especially of voluntary) influence from it ; and this is no 

 doubt true in a certain sense. Thus it is true that the transmission 

 of some influence up it to the brain is a necessary condition to the 

 experience of sensation, and that the transmission of some influ- 

 ence down it from the brain is a necessary antecedent to voluntary 

 motion ; but it does not by any means follow, as is often supposed, 

 that on this account sensation and voluntary action are to be 

 considered as really localized in the brain because of the necessity 

 of the intervention of that organ in order to their experience. 

 Though, no doubt, what are practically the centres for complicated 

 motions, are in the brain. 



With this proviso, we may treat, in the ordinary way, of the pat/is 

 which these influences seem to follow in traversing the spinal cord. 

 It is almost certain that both are predominantly situated in the white 

 fibres. It appears from recent experiments that both motor and 

 sensory impulses ascend and descend through the lateral columns, the 

 special functions of the anterior and posterior columns being unknown. 



* See ante, 16, p. 275. 



