652 SPECIAL SENSES. 



During sleep nearly all of the physiological processes, except those di- 

 rectly under the control of the sympathetic nervous system, are diminished 

 in activity. The circulation is slower, and the pulsations of the heart are 

 less frequent, as well as the respiratory movements. These points have 

 already been considered in connection with the physiology of circulation and 

 respiration. Physiologists have but little positive information with regard 

 to the relative activity of the processes of digestion, absorption and secretion, 

 during sleep. The drowsiness which many persons experience after a full 

 meal is probably due to a determination of blood to the alimentary canal and 

 a consequent diminution in the supply to the brain. 



CHAPTER XXL 



SPECIAL SENSES TOUCH, OLF ACTION AND GUSTATION. 



General characters of the special senses Muscular sense (so called) Sense of touch Variations in tactile 

 sensibility in different parts (sense of locality of impressions) Table of variations measured by the 

 sesthesiometer Appreciation of temperature Tactile centre Olfaction Nasal fossae Schneiderian 

 and olfactory membranes Olfactory (first nerve) Physiological anatomy Olfactory bulbs Olfactory 

 cells and terminations of the olfactory nerve-fibres Properties and uses of the olfactory nerves Mechan- 

 ism of olf action Relations of olf action to the sense of taste Reflex acts through the olfactory nerves 

 Olfactory centre Gustation Savors Nerves of taste Chorda tympani Glosso-pharyngeal (ninth 

 nerve) Physiological anatomy General properties of the glosso-pharyugeal Relations of the glosso- 

 pharyngeal nerves to gustation Mechanism of gustation Physiological anatomy of the organ of taste 

 Papillae of the tongue Taste-beakers Connections of the nerves with the organs of taste Taste- 

 centre. 



THE description of the nerves thus far has included motion and what is 

 known as general sensibility ; and knowledge of these properties of the nerv- 

 ous system has been derived mainly from experiments upon the inferior ani- 

 mals. As regards sensation, the experiments have referred to impressions 

 recognized as painful ; and these are conveyed to the centres by nerve-fila- 

 ments, anatomically as well as physiologically distinct from those which con- 

 vey to the contractile parts the impulses that give rise to motion. In regard 

 to the sensory nerves, simple impressions only have been described ; but it 

 is evident that the filaments of peripheral distribution of these nerves are 

 capable of receiving a variety of impressions, by which, to a certain extent, 

 the form, size, character of surface, density and temperature of objects are 

 recognized. There is also a general appreciation of heat and cold ; a sense 

 of resistance, which gives an idea of weight ; and finally, there are nerves of 

 peculiar properties, terminating in organs adapted to receive the impressions 

 of smell, taste, sight and hearing. 



The senses of olfaction, gustation, vision and audition belong to peculiar 

 organs, provided with nerves which have special properties and usually are 

 not endowed with general sensibility. These nerves have been omitted in 



