THE SENSES. 623 



qualities or conditions, not of external bodies but of the 

 nerves of sense themselves; and these qualities of the 

 nerves of sense are in all different, the nerve of each sense 

 having its own peculiar quality. 



There are two principal kinds of sensation, named 

 common and special. The first is the consequence of the 

 ordinary sensibilty or feeling possessed by most parts of 

 the body, and is manifested when a part is touched, or in 

 any ordinary manner is stimulated. According to the 

 stimulus, the mind perceives a sensation of heat, or cold, 

 of pain, of the contact of hard, soft, smooth, or rough 

 objects, etc. From this, also, in morbid states, the mind 

 perceives itching, tingling, burning, aching, and the like 

 sensations. In its greatest perfection, common sensibility 

 constitutes touch or tact. Touch is, indeed, usually classed 

 with the special senses, and will be considered in the same 

 group with them ; yet it differs from them in being a pro- 

 perty common to many nerves, e.g., all the sensitive spinal 

 nerves, the pneumogastric, glosso-pharyngeal, and fifth 

 cerebral nerves, and in its impressions being communicable 

 through many organs. 



Including the sense of touch, the special senses are five 

 in number, the senses of sight, hearing, smell, taste, 

 and touch. The manifestation of each of the first three 

 depends on the existence of a special nerve ; the optic for 

 the sense of sight, the auditory for that of hearing, and 

 the olfactory for that of smell. The sense of taste appears 

 to be a property common to branches of the fifth and of the 

 glosso-pharyugeal nerves. 



The senses, by virtue of the peculiar properties of their 

 several nerves, make us acquainted with the states of cur 

 own body ; and thus indirectly inform us of such qualities 

 and changes of external matter as can give rise to changes 

 in the condition of the nerves. That which through the 

 medium of our senses is actually perceived by the mind is, 

 indeed, merely a property or change of condition of our 



