314 THE CAT. [CHAP. ix. 



of the body presses on another. For the experience of sensation at 

 all the presence of a nerve is a necessary condition, and for the 

 exercise of special sensations we require special conditions of special 

 nerves. Sensations are not momentary, but persist more or less as 

 after effects, as may readily be perceived by first looking at the sun 

 or any very bright object, and then looking at any duller object, 

 when a dark spot, or spectrum, will be visible, resembling in shape 

 the bright object previously looked at. 



Frequent and continued repetition of the act of sensation diminishes 

 through exhaustion the sensitive power, but moderate, habitual 

 exercise of any faculty of sense tends to develope and perfect it. 



The special sensitive faculties are each sui generis, nor can one 

 merge in another, nor all in any common power of sensation. Thus, 

 before any special sense can exist or act, it must be present poten- 

 tially, i.e., it must be innate. Each special sense is elicited in a 

 manner totally unknown. If we count up all the number of sensa- 

 tions which differ in kind, their number will be seen to be consider- 

 able. Thus we have perception of change of temperature, the sense of 

 internal effort and resistance (the so-called muscular sense), the sense 

 of hunger, thirst, fatigue, sickness, and finally we have the two very 

 peculiar phenomena pleasure and pain, and the faculties, touch, 

 taste, smell, sight and hearing, commonly called the special senses 

 par excellence. All the special sensitive faculties, however, may be 

 deemed to have a relation of similarity with touch, and in each, as 

 we have seen, delicate nervous filaments are exposed in some special 

 manner to receive a stimulation of a peculiar kind. 



The feelings above enumerated are felt in different parts of the 

 body, nor are those persons who deny that they are really felt where 

 they seem to be felt, able to establish that they are actually felt in 

 any other place. 



As to pleasure and pain, they are feelings of a special and un- 

 analyzable kind which accompany (either one or the other) in a 

 greater or lesser degree all other feelings. The healthy performance 

 of the bodily functions, when felt at all, is pleasurable. Hindrances 

 to, or irregularities in, their performance are commonly painful. 

 With many exceptions and variations, it may be said that pleasura- 

 ble sensations tend to guide to actions which are profitable to the 

 organism, and painful ones to deter from such as are injurious. 



The SENSE OF TOUCH in its simplest form is a feeling of contact; 

 by intensifying this feeling we get feelings of resistance, density, 

 hardness, softness, &c. By employing movement in addition, size, 

 distance and figure become appreciated, and the nature of surfaces, as 

 e.g., their roughness, smoothness, &c. 



The delicacy of touch, as perceived by the skin, varies much in 

 different parts, as we have seen. These differences are doubtless 

 related to the differences in the number of nerve fibres supplied to 

 the different parts. 



Impressions received by touch become associated with other ones, 

 and cohere in more or less complex aggregations, which may be 



