CHAPTER XIX. 



THE SPECIAL SENSES: PRESSURE, TEMPERATURE, PAIN, MUSCLE- 

 SENSE, TASTE, HEARING, EQUILIBRIUM, VISION. 



IN the chapter on the Nervous System it was stated that the 

 result of the stimulation of a neurone depends not upon any 

 peculiarity of the neurone itself, but upon its anatomical rela- 

 tions to other neurones. For example, the neurones of which 

 the optic nerve is composed are not essentially different from 

 those which compose the trigeminal nerve, or from those which 

 compose the facial nerve ; but, as we will proceed to show, the 

 results and the methods of their stimulation differ according to 

 their anatomical relationships : 



1. The dendrones of the optic nerve terminate in the retinal 

 epithelium. This retinal epithelium is of such a nature that it 

 responds only to the stimulation of light falling into the eye. 

 The impulses thus aroused pass along the optic axones to the 

 central nervous system, where they connect with the dendrones 

 of other neurones situated in the cord, or in the brain, and 

 cause on the one hand reflexes, and on the other voluntary 

 movements accompanied by the phenomenon of consciousness. 



2. The dendrones of the trigeminal nerve, which supply the 

 skin of the face, terminate in various ways, so that some are 

 stimulated only by heat, some by cold, some by pressure, and 

 the impulses thus aroused pass to the central nervous system 

 along axones which have connections similar to those of the 

 optic nerve. 



3. The dendrones of the facial nerve lie within the central 

 nervous system, and they are normally stimulated by impulses 

 which pass to them from other neurones in the brain or spinal 

 cord. These impulses they transmit along their axones which 

 terminate in the muscles of the face, and which are thus, volun- 



222 



