THE SENSE OF TOUCH 631 



delirium, and may take the form of animals such as cats, rats, or creeping 

 loathsome forms, etc. 



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 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 sensation of a 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 disa- 

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

 disagreeable to us. It is evident, however, that on this habit of referring our 

 sensations to causes outside ourselves, perception, depends the reality of 

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

 of touch and sight. By the co-operation 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 confi- 

 dence 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. 



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

 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- 



