

INTERNAL OR CORPOREAL SENSATIONS. 347 



observed which require for their explanation the supposition of the existence 

 of any additional sense ; so that, not only are we unable, from the nature of 

 things, to prove the existence of additional senses in animals, but their beha- 

 vior does not justify the inference that such senses are present in them. 



The utility of each sense is very greatly enhanced by the numerous 

 qualitative sensations which we are enabled to experience through it ; 

 such, for example, as the almost endless distinctions of light and 

 shadow, color and hue, force, pitch and timbre of sound, and the nu- 

 merous varieties of odors and tastes. The qualities of sensation per- 

 ceived through the same sense are, indeed, most diverse. The causes 

 of the qualitative differences of color and of sound are closely related, 

 being, in both, dependent on definite numerical relations between the 

 numbers of the luminous or sonorous undulations ; but, as regards 

 smell and taste, no such relations have yet been established. 



Pain. 



When any of these external senses, or senses responding to external 

 causes, are unduly excited by their proper stimuli highly intensified, 

 pain is the result, as is shown by the effect of gazing at the sun, or at 

 any other extremely vivid light, especially if long continued, and also 

 by the effects of shrill, loud, and grating noises ; but the pain, in each 

 of these cases, is peculiar. That which is caused by acrid smells, very 

 pungent tastes, the action of irritants on the naked cutis after blisters, 

 the pressure of sharp points or heavy weights, and by bodies in rapid 

 motion, in the organs of smell, taste, and touch, is due to excessive 

 stimulation of the common sensibility. The occurrence of pain from 

 the over-excitement of the special senses, indicates their relation to 

 this common sensibility. It is said, however, that powerful mechanical 

 stimulation of the optic nerve causes no pain, but intense luminous sen- 

 sations ; this is probably dependent on the still existing connection of 

 the nerve with at least one eyeball. But it is the common sensibility 

 which is ordinarily excited in the feeling of pain. Such pain is not 

 merely an intensified normal sensation, for very hot bodies cause a 

 painful, and not an exalted thermal sensation. The common sensi- 

 bility is also actively concerned in those various sensations which in- 

 form us of the conditions of the nervous centres, produced, not by ex- 

 ternal stimuli, giving us a knowledge of external objects and forces, 

 but by internal states of the organism ; for these so-called internal 

 sensations, some of which are pleasurable and some painful, seem to 

 be distinct modifications of common sensation. 



Internal or Corporeal Sensations. 



These internal sensations, though dependent, as just stated, on con- 

 ditions of the nervous system, are, like the external sensations, usually 

 referred to special seats, that is, to certain tissues or organs of the 

 body, and hence may be called corporeal. Some are referred to the 

 organs of animal life, and others to the organs of vegetative life. 



Of the former, the muscular sensations are the most important ; the 



