SENSATION IN GENERAL. 337 



corporeal. The ordinary external objective stimuli are physical, ma- 

 terial, or mechanical, chemical, thermal, or electrical, and include 

 matter of all kinds, all forms of motion, molar or molecular, undula- 

 tory or impulsive, such as those which produce sound, heat, perhaps 

 chemical action, light, and electricity. Such motions, succeeding 

 each other according to fixed laws, may be supposed to be propagated 

 onwards, through the sensory apparatus, or to produce corresponding 

 motions, molar or molecular, in it. The internal or corporeal objec- 

 tive stimuli reside in the blood, such as its temperature, and peculiari- 

 ties in its chemical composition; or they may proceed from the tissues 

 to which the nerves are distributed, changes in the nutritive metamor- 

 phosis of which may affect the nerves; or they may depend upon 

 changes in the nutritive condition of the extremities, or of the trunks, 

 of the sensory nerve-fibres, or of the sensorial nervous centres them- 

 selves. The sensations resulting from such stimuli are commonly 

 known as subjective. But, as we have already mentioned, the only 

 true subjective stimuli which can cause sensations are those depending 

 on purely psychical or mental states, such as ideas or emotions. It is 

 often difficult, in regard to certain sensations, to say whether they are 

 corporeally or externally objective ; also whether they are truly sub- 

 jective, corporeally objective, or externally objective. Experience 

 and close examination can alone decide these points. Sensory stimuli 

 have also been defined, according to whether they are able to produce 

 only one kind of sensation, or several different kinds, as either homol- 

 ogous or heterologous. ' For the reception of the former, such as light 

 and sound, the sensory organ requires to be peculiarly constructed, 

 and the nerve to be specially sensitive at its extremity. The latter, 

 such as electricity or mechanical shocks, produce various forms of 

 sensation, and act on all kinds of sensory organs. The sensations 

 produced by either kind of stimulus are similar for each organ. A 

 homologous stimulus acts only on its proper nerve; light, for example, 

 has no effect on the nerves of taste; moreover, such a stimulus acts 

 only on the periphery of the nerve-fibres, and not upon their trunks, 

 as is illustrated by the fact that the optic nerve itself is insensible to 

 light. As different stimuli, acting on the same sensory organ, give 

 rise to the same kind of sensation; and again, as the same stimulus 

 may produce different sensations, if it acts on different organs, it 

 would appear that each sensory apparatus has its own recipient power 

 or endowment, perhaps likewise some special energy in its nerve, and, 

 as is generally supposed, in its proper sensorial nervous centre. 

 Hence, an absolutely deaf person cannot hear even the loudest sonor- 

 ous vibrations; but he may perceive them, through the sense of touch, 

 as physical vibrations of matter. 



Certain general facts, in regard to sensation, have been described 

 as laws of sensation. For example, in the ordinary act of external 

 objective sensation, the surface of the sensory organ is the immediate 

 recipient of the stimulus, and,- through it, the first impression is made 

 on the peripheral extremities of the sensory nerve-fibres. These latter, 

 then, play an internuncial part, and conduct the changes induced in 

 them, to the sensorial nervous centre, which itself undergoes changes, 



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