254 'ON THE SENSES. 



light, when tliey strike upon this terminal apparatus of the optic nerves, produce 

 in its substance a chemical change, and thus supply a chemical irritant, a con- 

 jecture not without support in the many well-known chem'cal effects of light, 

 though not susceptible of previous proof. The nervous current excited by these 

 different mediate or immediate irritants traverses the nerves, an^, like the elec- 

 tric current of the wires, is adapted to the production of the most diversified 

 effects according to the organization and apparatus with which the nervous fibre 

 is in connexion. There is a large class of these filaments which, having their 

 origin in the brain and spinal column, penetrate the flesh, and, in some manner 

 not clearly understood, connect themselves with its elements, the muscular fibres. 

 If a current be excited by some irritant in a nerve of this sort, that current will 

 flow from the point of irritation to both extremities of the fibre; at the outward 

 extremity it takes effect upon the muscular fibre, which is so constituted that it 

 necessarily contracts i;nder the operation of the nervous current. The part of 

 the current which flows to the inner extremity in the brain and spine meets 

 there with no arrangement through which any phenomenal effect is manifested ; 

 on the other hand, there exists at the inner extremity of every nerve proceeding 

 to a muscle, an apparatus through which the will can excite a current in the 

 particular fibre. The microscope shows us this apparatus in the form of a small 

 bulbous body occupied by a turbid fluid and a nucleus, and physiology teaches 

 US that through this bulb the physical energy acts upon the nervous fibre ; but 

 though we thus learn the state and purpose of the mechanism, not the slightest 

 intimation do we gather of the nature of its action, of the way and manner in 

 which through this bulb the presumed immaterial principle of the will communi- 

 cates to the fibre the impulse from which results the contraction of the muscle 

 and the varied movements of the limbs. This riddle we shall, perhaps, never 

 solve ; it would still be one if the process within the active fibre of the nerve 

 were laid open ; nay, could physiology surprise the vital activity of the terminal 

 bulb itself, the problem would still remain inexplicable so long as the will, the 

 last term of the causative series, subsists as a force independent of matter. The 

 riddle only becomes more wonderful when we see that, at the inner end of most 

 probably all the remaining nerve fibres in the brain and spine, are found the so- 

 called ncn-e cells, being bulbous bodies, existing under wholly similar conditions. 

 We distinguish from the nerves of motion, whose destination we have been con- 

 sidering, a second great class of nervous fibres as nerves of sensation, being all 

 those whose excitation, by whateveiv irritant it be determined, produces some 

 sort of sensation. Their fibres fully resemble those which effect the movements 

 of the limbs, and here also we have to seek the nature of the apparatus at both 

 extremities, as well as that which communicates the excitation as that through 

 which the latter produces an effect. 



We find the nerves of sensation stretching between the brain and spine on the 

 one hand, and the exterior surface of the body, as well as almost all the internal 

 organs, on the oth(u-. The apparatus of the inner extremity, therefore, is seated 

 in the central part of the nervous system, that of the outer extremity in the 

 organs of sense, the whole external surface of the skin, and in all sensitive in- 

 ternal organs, that is in all parts which yield a sensation on being wounded or 

 in-itated The destination of the terminal apparatus is here, however, reversed, 

 as compared with the nerves of the muscles, inasmuch as in the nerves of sen- 

 sation the appai-atus through which their current produces its sensitive effect is 

 placed at the inner extremity in the spine and brain, while the organization 

 connected with the outer extremity is destined to render the nerve susceptible 

 of excitation by certain external agents. If we irritate a nerve of sensation, the 

 optic nerve for instance, at any point of its course, as by compressing or electri- 

 fying it, a current flows to both ends of the irritated nerve: that which arrives 

 at the inner extremity takes effect on the appropriate apparatus and produces in 

 it a wholly unknown condition, whose result, as regards consciousness, is a sen- 



