1879.] on Sensation. 121 



have noticed tlio extreme delicacy of the sensations produced by tlie 

 contact of bodies with the ends of the hairs of the head ; and the 

 " whiskers " of cats owe their functional importance to the abundant 

 supply of nerves to the follicles in which their bases are lodgcrl. 

 What part, if any, the so-called "tactile corimscles," "end bulbs," 

 and " Pacinian bodies " play in the mechanism of touch is unknown. 

 If they are sense organs, they are exceptional in character, in so far 

 as they do not appear to be modifications of the epidermis. Nothing 

 is known resjiecting the sense organs of those sensations of resistance 

 which are grouped under the head of the muscular sense ; nor of the 

 sensations of wvarmth and cold ; nor of that very singular sensation 

 which we call tickling. 



In the case of heat and cold, the organism not only becomes affected 

 by external bodies, far more remote than those which affect the sense 

 of smell ; but the Democritic hypothesis is obviously no longer per- 

 missible. When the direct rays of the sun fall u23on the skin, the 

 sensation of heat is certainly not caused by " attenuated films " thrown 

 off from that luminary, but to a mode of motion which is transmitted 

 to us. In Aristotelian phrase, it is the form without the matter of the 

 sun which stamjjs the sense organ ; and this, translated into modern 

 language, means nearly the same thing as Hartley's vibrations. Thus 

 we are prepared for what happens in the case of the auditory and the 

 visual senses. For neither the ear nor the eye receives anything but 

 the impulses or vibrations originated by sonorous or luminous bodies. 

 Nevertheless, the receptive apparatus still consists of nothing but 

 specially modified epithelial cells. In the labyrinth of the ear of the 

 higher animals the free ends of these cells terminate in excessively 

 delicate hair-like filaments; while, in the lower forms of auditory 

 organ, its free surface is beset with delicate hairs like those of the 

 surface of the body, and the transmissive nerves are connected with 

 the bases of these hairs. Thus there is an insensible gradation in 

 the forms of the receptive apparatus, from the organ of touch, on the 

 one hand, to those of taste and smell; and, on the other hand, to 

 that of hearing. Even in the case of the most refined of all the 

 sense organs, that of vision, the receptive apparatus departs but little 

 from the general type. The only essential constituent of the visual 

 sense organ is the retina, which forms so small a part of the eyes of 

 the higher animals ; and the simplest eyes are nothing but portions of 

 the integument, in which the cells of the epidermis have become 

 converted into glassy rod-like retinal corpuscles. The outer ends of 

 these are turned towards the light ; their sides are more or less exten- 

 sively coated with a dark pigment, and their inner ends are connected 

 with the transmissive nerve fibres. The light impinging on these visual 

 rods produces a change in them which is communicated to the nerve 

 fibres, and, being transmitted to the sensorium, gives rise to the sensa- 

 tion — if indeed all animals which possess eyes are endowed with what 

 we understand as sensation. 



Vol. IX. (No. 71.) k 



