SECTION A III 



SPECIAL SENSE ORGANS 



Revised for the Fifth Edition 

 Bt DAVID WATERSTON, M.A.,M.D., F.R.C.S.E., King's College, London 



PROFESSOR OF ANATOMY IN THE UNIVERSITY OF LONDON 



GENERAL CONSIDERATIONS 



THE term "special sense organs" indicates those structures situated on or 

 near the surface of the body which receive the impressions of sound, light 

 taste and smell, and transmit them to the brain in the form of nerve 

 impulses. 



The essential difference between what is termed general sensibility and the 

 special senses lies in the fact that the organs of special sense are each sensitive to 

 a specific stimulus which does not affect the general sensory apparatus of the 

 body surface to an appreciable degree. 



Thus, the waves of light or of sound, flavoured substances which have a taste, and the 

 minute particles which stimulate the sensory organ for smell — all these varied stimuli create 

 no impression when they come into contact with the sensitive general surface of the body. 



The vibration of sound waves present in an organ pipe may indeed be felt by the hand, but 

 the sensation is that of vibration and not of sound. 



This difference in function between the ordinary and the special senses 

 as well as the difference between the individual organs of special sense, is as- 

 sociated with a difference in structure; for each special sense organ has a charac- 

 teristic receptive mechanism of cells highly specialised in form and structure, 

 which receive the stimuli coming from without, and transmit them to the brain in 

 the form of a nerve-current. These cells may be derived by the specialisation of 

 certain cells coming directly from the surface of the body, or they may be cells 

 derived from the central nervous system — as in the case of the eye. In this case, 

 the cells are placed in close relation to the terminals of a special cranial nerve. 



Many of the sense organs, and especially the eye and ear, are highly com- 

 plex in structure. The complexity is due largely to the elaborate mechanical 

 arrangement for receiving the external stimulus, and for conveying it to, or 

 focussing it upon, the sensory cells proper. 



It must always be borne in mind that sensation itself is a function of the brain — it is the 

 response in consciousness to the afferent impressions transmitted to the brain by the sensory 

 nerves. Further, the quality of the sensation does not arise in the sense organ, but in the brain 

 itself. Thus, stimulation of the trunk of the optic nerve by mechanical means produces 

 sensations of hght, apart from stimulation of the retina. 



In the following account, the organs of smell, taste, vision and hearing will be 

 successively considered. 



I. THE OLFACTORY ORGAN 



The olfactory apparatus [organon olfactus] in man does not reach the high 

 development which is found in many of the lower animals. In them, not only is 

 the sensory apparatus found distributed over a large area of the nasal mucous 

 membrane, but the central connections of the olfactory nerves make up a 

 considerable portion of the brain, including all those structures known under the 

 name of rhinencephalon. In man, sensibility to smell is localised to a compara- 

 tively limited area in the upper part of the nasal cavity, known as the olfactory 

 area. 



The structure of the nose in all its parts has been fully dealt with in the 



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