426 Dynamic Theory. 



clothes fit well and are not heavy, we may become quite unconscious of 

 their contact with us. But if they press some spot, or cause too much 

 warmth, we become conscious of the action of the senses of pressure 

 and temperature. Many of the touch sensations are compound, involv- 

 ing some two of the three simple ones as friction and pressure, fric- 

 tion and heat, pressure and heat or all three. A stimulus may ex- 

 haust itself in the cuticle alone, and be propagated thence, through the 

 nerves as nervous electricity, to the brain. If the stimulus is exces- 

 sive, the excess, after exciting the different nerves, is expended on the 

 papillary layer of the dermis, causing increased secretion from its blood 

 vessels. Scalding heat causes an effusion of serum, which separates 

 the dermis from the cuticle and makes a blister. Excessive pressure 

 produces the same effect. The sensation in such cases is simply one of 

 pain, which is the only sensation arising from the agitation of the nerves 

 of the papillae directly they not being the organs of touch, but only 

 the vehicles of the stimulating current. Gentle pressure, or the press- 

 ure of a soft body over a great surface of the skin, acting through the 

 cuticle upon the papillary layer, is not painful unless protracted, but 

 stimulates the vascular system of the dermis to supply more nourish- 

 ment to the cells of the mucous layer of the epidermis squeezes out 

 the juice into the cuticle, as it were, and builds it up more rapidly, and 

 so erects a barricade between the tender papillary la} r er and the pressing 

 force outside. Thus originate the extra thicknesses of epidermis on the 

 soles and palm, and the callosities formed in chance spots exposed to 

 protracted pressure. 



The different stimuli, friction, heat and pressure, when they are ex- 

 pended on the epidermis, are converted, in part, into afferent nerve 

 currents, which affect the brain cells corresponding to touch sensation, 

 and the rest raises the molecular action of the tissues of the skin and ad- 

 jacent parts, giving an increase of temperature, which, if strong enough, 

 also produces a sensation in the brain. Evidently, different sets of 

 nerve fibres must be devoted to the carrying of the different sorts of 

 sensations. In the case of the eye we shall see that there are various 

 sets of rods and cones to convey the stimuli of the various light rays 

 composing the spectrum, that we can perceive. Radiant heat is gov- 

 erned by the same laws as light, consisting simply of lower tones of the 

 same radiant energy. Analogy would lead us to suspect that the or- 

 gans of heat sensation might be constructed on the same plan a differ- 

 ent set of organs for different rates of radiant vibration. But it ap- 

 pears to be not so. For all the rays of heat appear to our conscious- 

 ness as of the same quality ; as if the wave lengths were all the same, 

 and the differences consisted only in amplitude or intensity. But al- 

 though the energy of the dark radiation, or heat^ of an incandescent 



