4 PHYSIOLOGY CHAP. 



ceptible to one particular stimulus while absolutely or relatively 

 inexcitable to stimuli of .other kinds. It is not a sufficient 

 explanation of this fact to say that in certain cases the influence 

 of the inadequate stimuli is hindered or impeded by the topo- 

 graphical position of the sense-organs. The auditory apparatus, 

 for example, is shielded from the action of light, of mechanical 

 impacts, of various vapours ; the visual apparatus is well protected 

 from mechanical and chemical stimuli. On the other hand, the 

 auditory cells are perfectly accessible to sound-waves, the visual 

 cells to light, the olfactory cells to the air inspired, the gustatory 

 cells to the food-stuffs ingested. These statements, which neglect 

 the internal constitution of the sense-organs, fail to explain the 

 specific susceptibility of the latter to given stimuli. What is the 

 intrinsic organic condition that prevents the peripheral organs of 

 taste and smell from reacting to light, warmth, or mechanical 

 pressure (which are adequate stimuli for the visual and cutaneous 

 nerves), while they are excessively sensitive to certain chemical 

 stimuli ? From the teleological point of view, we know this must 

 be so. If it were otherwise, if the organs of taste and smell were 

 not specifically predisposed to react to chemical stimuli, but 

 reacted with the same facility as the skin to heat, contact, 

 and pressure, they would be incapable of conveying to our 

 consciousness any precise intimation of the nature of the chemical 

 stimuli to which they are adapted. The same may be said of the 

 specific adaptation of the retina to light, the cochlea to sound, etc. 

 But even if the teleological connection between the specific nature 

 of the sense-organs and their specific function is plain, we are still 

 ignorant as to the internal nature of their respective structures, 

 on which depends their specific excitability to different kinds of 

 stimuli. In all probability, as Tick assures us, there are in the 

 peripheral sense-organs compounds of a highly unstable molecular 

 constitution, which are decomposed by slight impacts, and thus 

 develop energy which acts on the nerve as an effective stimulus. 



The senses can be distinguished and classified either by their 

 anatomical situation, or by the nature or quality of the stimulus 

 adequate to excite them, or lastly, by the kind of sensation which 

 they arouse in consciousness. These different categories mostly 

 coincide. Thus vision is the sense of the eye, for which the 

 adequate stimulus is light, which produces visual sensations ; 

 hearing is the sense of the ear, and is excited by tones and noises 

 which arouse auditory sensations in consciousness ; taste is the 

 sense of the tongue, and is excited by sapid substances that arouse 

 gustatory sensations ; smell is the sense of the nose, and is excited 

 by odorous substances which evoke olfactory sensations. But 

 when we apply the anatomical test we must further distinguish a 

 cutaneous sense, a muscular sense (inclusive of tendons and joints), 

 and a visceral sense ; according to the nature of the stimulus, we 



