CHAPTER X. SENSE-ORGANS. 



1. Sensation. We now turn to the chief sense-organs 

 to see what their structure is, and how they are enabled 

 to translate external forces into nerve-impulses which 

 produce sensations in the brain. We say " translate" and 

 it is important to fully realize that the nerve-impulse is 

 a totally different thing from the external force which 

 rouses it, with which it does not correspond either quantita- 

 tively or qualitatively. We have no reason to doubt that a 

 nerve-impulse travelling along the fibres of the auditory 

 nerve is precisely the same as a nerve-impulse travelling 

 along the fibres of the optic nerve. The essential differences 

 in the two cases are, firstly, in the capacity of one sense- 

 organ to be stimulated by air-waves, and of the other to be 

 stimulated by ethereal waves ; and secondly, in the capacity 

 of one part of the brain to produce the consciousness we 

 call " light " when stimulated, and of the other to produce 

 the consciousness we call " sound." If we stimulate the 

 optic nerve-fibres in any way, the same sensation of light is 

 produced in the brain, as the familiar experience of seeing 

 coloured patterns when the closed eye is pressed upon shows. 



2. Simplest Sense-organs. The simplest kind of sensory 

 nerve-ending is a simple arborization under the epidermis : 

 only vague sensations are obtained through the help of this. 

 For the definite sense of touch to be effected the arboriza- 

 tions are ensheathed in a series of layers of cellular con- 

 nective tissue, the whole forming a somewhat bulbous body 

 (see fig. 13) to which various names are applied, according 

 to the details of structure (tactile corpuscles, end-bulbs, 

 Pacinian corpuscles, etc.). In the other important sense- 

 organs we find a specially modified epithelium as the organ 

 directly affected by external agents. Around the bases of 

 the cells of this epithelium the sensory nerve-fibres form an 

 arborization. 



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