340 SPECIAL PHYSIOLOGY. 



nerves. The duration of sensory impressions on the skin, depends 

 chiefly, on the length of time, during which the particular stimulating 

 body has been applied to it ; the instantaneous prick of a pin leaving 

 an evanescent sensation, whilst the longer contact of the same body is 

 followed by a more lasting effect. Sensations occurring in debilitated 

 states of the health, or in weakened conditions of the nutrition of the 

 nerves and nervous centres, are more durable than those occurring in 

 the healthy state; in the latter case the nutrient effort of the nervous 

 substance more quickly restores it from the peculiar condition of 

 change produced by its stimulation. 



After-sensations, more or less distinct, are noticed in regard to all 

 the senses. They depend on the persistence of the disturbed state of the 

 sensory apparatus, nerve, or sensorial nervous centre, which parts are 

 not immediately restored to a condition of rest or equilibrium. These 

 after-effects endure longer in enfeebled states of the sensory appa- 

 ratus, or in debilitated states of the nervous substance, as illustrated 

 in the case of ocular spectra, which remain longer on the retina in 

 persons in whom that part of the eye is enfeebled by age or other 

 causes. Ocular spectra recur, as well as persist. 



Certain sensations are accompanied by other or associated sensa- 

 tions. These arise either from simple radiation of the same sensation, 

 as in the extension of a luminous impression on the retina, and the 

 diffusion of a painful sensation through inflamed or excited nerves, as 

 in neuralgia of the face from toothache ; or from the excitation of a 

 different sensation, as when a cold or creeping sensation is caused 

 along the back, or the teeth are set on edge by disagreeable jarring 

 sounds, like that produced by drawing a slate pencil vertically upon a 

 slate, or by tearing silk or calico. 



The repetition, if moderate, of a sensation, has the effect of strength- 

 ening the power of the sensory organ, nerve, or centre, or of all three 

 parts. This is illustrated by the exercise of a particular sense, as, 

 e. g., of the eye by sailors, of the ear by the practised musician, of taste 

 and smell by wine- and tea-tasters, and of the sense of touch by watch- 

 makers, silk- weavers, and others. But the too frequent repetition of 

 'powerful sensations exhausts part of, or all the sensorial apparatus. 

 It is also to be observed, that whilst the repetition of the same sen- 

 sory impressions may strengthen the sensory faculty, it at the same 

 time weakens the power of attention to such sensations, especially if 

 these be monotonous, like the ticking of a clock, or the clack of a 

 mill ; and thus persons become habituated to sounds, and also to con- 

 tinued odors, whether agreeable or disagreeable, without attending to 

 or perceiving them. 



The alternation of different sensations, or of sensory impressions of 

 the same kind, but differing in degree, is a most effectual mode of 

 educating the senses. 



Remarkable individual peculiarities, quite within the limits of health, 

 are observable in different persons, as regards their respective powers 

 of sensation. Some of these may be due to education, training, or 

 habit ; but many are only to be explained by reference to individual 

 peculiarities of endowment. For example, there are persons unable 



