256 ON THE SENSES. 



tion goes, it reveals to ng, at the end of all the nerves of sensation in the brain 

 and spinal marrow, organizations in which we seek in vain to distinguish any 

 essential difference. Everywhere, there are seen the seemingly simple nucleated 

 bulbs which we have designated as nerve-cells, and singularly enough they are 

 to appearance the same bulbs which we find at the inner extremity of the nerves 

 of motion, but which, as regards the latter, we have learned to consider as or- 

 ganisms for producing excitation, not as in the case of nerves of sensation or- 

 ganisms upon which the excited nerve is destined to operate, and through which 

 its phenomenal action is manifested. And yet differences must exist, since it 

 is incredible, even upon the most strictly spiritualistic principles, that an imma- 

 terial soul should be capable, from the same processes in the nervous matter, of 

 creating for itself different sensations. The procedure by which a sensation of 

 light is produced in the mind must of necessity be different from that by which 

 a sensation of sound or of touch is provoked. If we can detect no such differ- 

 ences in the terminal apparatus of the nerves, the fault lies with our present de- 

 fective means of investigation, and we must await further light from future 

 researches. 



Having thus seen that a numerous class of nerves are adapted by the nature 

 of their inner terminal apparatus to become the vehicles of sensation, it remains 

 for us to cast a glance at their outer terminations and the arrangements there 

 provided. Here we find those wonderful adaptations, the organs of sense, of 

 whose destination something has already been said. The capacity of the optic 

 nerve, by virtue of its interior apparatus, to produce on being excited the per- 

 ception of light, would be of little account if the nerve lay bare to the day, so 

 as to respond by a sensation of light at the solicitation of every casual excite- 

 ment, whether proceeding from a blow, from heat, cold, electricity, or chemical 

 agency. It becomes, however, of inestimable value when the nerve, besides 

 being adapted to one only definite kind of excitement, is carefully sheltered 

 from eveiy other, and when this specific excitement is one to which all other 

 nerves are insensible. The optic nerve is destined for excitation through the 

 undulations of light, and is provided at the end directed towards the outward 

 world with the organs necessary for that purpose. Those undulations form na- 

 turally its sole and exclusive means of excitation, and are hence designated in 

 science as the "adequate" irritant of the optic nerve. Only thus is it possible 

 for the excitability of this nerve to convey to the sensoriura an authentic im- 

 pression of the outw'ard world, so far as it is to be derived from the definite ex- 

 ternal agent, light. But how imperfect were the communications of the visual 

 nerve if it simply apprised us of the presence and absence of light, and, perhaps, 

 through the degi'ee of excitation, of the intensity of its beams, if the only 

 function of our eyes were to distinguish daylight and darkness ! How vastly 

 is their value enhanced through their power of placing external objects before 

 us in an endless diversity of shapes, magnitudes, and colors ! We can here 

 only depict with a few strokes the principle of those exquisite performances 

 which we hope hereafter to discuss in a more detailed manner. The first con- 

 dition of this action is the capacity of the optic nerves to produce those differ- 

 ently qualified sensations which correspond to the different sorts of external 

 light, that is to say, to the waves of different lengths formed by the vibrating 

 particles of light, and which induce in our minds the perception of different 

 colors. Each perception of color corresponds to a wave of the vibi-ating light- 

 element of determinate length. That through this faculty alone the circle of 

 intimations which the mind receives by means of the nerves of vision is greatly 

 widened is at once evident. But as thus far we have spoken of one nervous 

 current in general, as of one like process in all nerves, it is now proper to sug- 

 gest a slight modification of this expression, without thereby vitiating, however, 

 our comparison of the electric and the nervous currents. It is, indeed, very con- 

 ceivable, and even probable, that the "current" in the nerves of motion which 



