SENSATION AND SENSIFEROUS ORGANS. 97 



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 modi- 

 fications of the epidermis. Nothing is known respecting the sense- 

 organs of those sensations of resistance which are grouped under the 

 head of the muscular sense ; nor of the sensations of warmth 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 permis- 

 sible. When the direct rays of the sun fall upon 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 

 stamps the sense-organ ; and this, translated into modern language, 

 means nearly the same thing as Hartley's vibrations. Thus we are pre- 

 pared for what happens in the case of the auditory and the visual 

 senses. For neither the ear nor the eye receives anything but the im- 

 pulses or vibrations originated by sonorous or luminous bodies. Never- 

 theless, the receptive apparatus still consists of nothing but specially 

 modified epithelial cells. In the labyrinth of the ear of the higher ani- 

 mals the free ends of these cells terminate in excessively delicate hair- 

 like filaments ; while, in the lower forms of auditory organ, its free sur- 

 face 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 ap- 

 paratus, 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 epi- 

 dermis have become converted into glassy, rod-like retinal corpuscles. 

 The outer ends of these are turned toward the light ; their sides are 

 more or less extensively coated with a dark pigment, and their inner 

 ends are connected with the transmissive nerve-fibers. The light im- 

 pinging on these visual rods produces a change in them which is com- 

 municated to the nerve-fibers, and, being transmitted to the sensorium, 

 gives rise to the sensation if indeed all animals which possess eyes are 

 endowed with what we understand as sensation. 



In the higher animals, a complicated apparatus of lenses, arranged 

 on the principle of a camera obscura, serves at once to concentrate 

 and to individualize the pencils of light proceeding from external 

 bodies. But the essential part of the organ of vision is still a layer of 

 cells which have the form of rods with truncated or conical ends. By 

 what seems a strange anomaly, however, the glassy ends of these 

 vol. xv. 1 



