SENSE PERCEPTIONS 597 



One uniform internal cause, which may act on all the nerves of the senses 

 in the same manner, is capillary congestion. This one cause excites in the 

 retina, while the eyes are closed, the sensations of light and luminous flashes; 

 in the auditory nerve, the sensation of humming and ringing sounds; in the 

 olfactory nerve, the sense of odors; and in the nerves of feeling, the sensation 

 of pain. In the same way a chemical substance introduced into the blood 

 may excite in the nerves of each sense peculiar symptoms: In the optic nerves, 

 the appearance of luminous sparks before the eyes; in the auditory nerves, 

 tinnitus aurium; and in the common sensory nerves, the sensations of creeping 

 over the surface. So, also, among external causes, the stimulus of electricity, 

 or the mechanical influence of a blow, concussion, or pressure, excites in the 

 eye the sensation of light and colors; in the ear, a sense of a loud sound or 

 of ringing; and in the tongue, a saline or acid taste. 



Sense Perceptions. The habit of constantly referring our sensa- 

 tions to external causes leads us to interpret the various modifications which 

 external objects produce in our sensations, as properties of the external bodies 

 themselves. Thus we speak of certain substances as possessing a disagreeable 

 taste and smell; whereas, the fact is their taste and smell are only disagree- 

 able to us. It is evident, however, that on this habit of referring our sensa- 

 tions to causes outside ourselves, perception, depends the reality of the ex- 

 ternal world to us; and more especially is this the case with the senses of 

 touch and sight. By the cooperation of these two senses, aided by the others, 

 we are enabled gradually to attain a knowledge of external objects which 

 daily experience confirms, until we come to place unbounded confidence in 

 what is termed the evidence of the senses. 



We must draw a distinction between mere sensations, and the judgments 

 based, often unconsciously, upon them. Thus, in looking at a near object, 

 we unconsciously estimate its distance and say it seems to be ten or twelve 

 feet off. But the estimate of its distance is in reality a judgment based on 

 many things besides the appearance of the object itself; among which may 

 be mentioned the number of intervening objects, the number of steps which 

 from past experience we know we must take before we could touch it, and 

 many others. 



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I. THE SENSES OF TOUCH, PAIN, TEMPERATURE, AND 

 THE MUSCLE SENSE. 



The Sense of Touch. The sense of touch, like all the special 

 senses, possesses a special end-organ for the initiation of a nerve impulse 

 through contact with external objects. The sense organ of touch is not con- 

 fined to particular parts of the body of small extent, like the organ of sight, 

 for example, but is found in all parts of the skin and its inversions, the stomo- 

 cleum and proctodeum. The nerves of sensation are contained in the same 



