234 INNERVATION. [CHAP. IX. 



one or more of its staminal principles, or vitiated by some morbid 

 element. 



The nervous trunks as they exist in different regions are usually 

 compound; that is, they contain fibres of different endowments. 

 In some situations, it is true, the fibres of one kind predominate 

 so much as to give the trunk the physiological character which 

 belongs to them; but it likewise enjoys, in a proportionate degree, 

 the functions of those fibres, which are few in number. For exam- 

 ple, the facial nerve, or portio dura of the seventh pair, is called 

 motor, because it is almost wholly composed of motor fibres ; but it 

 contains, besides, in very much smaller number, some sensitive fila- 

 ments, which it probably derives from anastomoses with neighbour- 

 ing nerves. The third, fourth, and sixth pairs of nerves may be 

 quoted as of similar constitution to the facial. In the ramifications 

 of the fifth nerve, on the other hand, the filaments of sensation are 

 predominant; those of motion being much fewer, and confined to 

 the branches of its inferior maxillary division. 



It is at the points of emergence of the roots of the nerves from 

 the nervous centres that we find the most complete isolation of 

 function. This is well exemplified in the spinal nerves and in the 

 fifth pair. These nerves emerge from their respective centres by 

 two bundles of fibres, of which one is sensitive, the other motor ; 

 the former having almost always the distinctive features of greater 

 size than the latter, and of having a ganglion formed upon it. But, 

 even in these instances, it has lately been made a matter of question 

 whether the smaller root, which experiment has satisfactorily shewn 

 to be motor in its function, does not also contain a very slight pro- 

 portion of sensitive filaments. 



The stimuli by which the action of nerves is ordinarily pro- 

 voked are of two kinds, mental and physical. In all voluntary 

 actions, an act of the mind is the excitant of the nerve. Sensa- 

 tions are caused by the influence of physical agents upon nerves, 

 which communicate with the sensorium commune. The change in 

 the nerve, by reason of this communication, gives rise to a corre- 

 sponding affection of the mind. It is wonderful how quickly such 

 changes are propagated, and with what precision they are perceived 

 by the mind, although the physical excitant may itself be a fine 

 point invisible to the naked eye, applied with the slightest force, 

 and coming in contact with a spot equally difficult of apprecia- 

 tion. If the communication between the nerve and the centre be 

 cut off, the will can exert no influence upon the muscles supplied 

 by the nerve below the section ; nor will the mind perceive any 



