THE SENSES. 659 



cases may be mentioned, humming and buzzing in the ears caused by 

 some irritation of the auditory nerve or centre, and even musical sounds 

 and voices (sometimes termed auditory spectra) ; also so-called optical 

 illusions: objects are described as seen, although not present. Such 

 illusions are most strikingly exemplified in cases of delirium tremens or 

 other forms of delirium, and may take the form of cats, rats, creeping 

 loathsome forms, etc. 



Causes of Illusions. 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 sensa- 

 ions 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, also, a narcotic substance introduced into the blood, excites in the 

 nerves of each sense peculiar symptoms: in the optic nerves, the appear- 

 ance 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 elec- 

 tricity, 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 aloud sound or of ringing; in the tongue, a saline or acid taste; and 

 in the other parts of the body, a perception of peculiar jarring or of the 

 mechanical impression, or a shock like it. 



Experiments seem to have proved, however, that none of the nerves 

 of special sense possess the faculty of common sensibility. 



Perceptions. The habit of constantly referring our sensations to ex- 

 ternal 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 disagreeable to us. It is evident, however, that on this habit 

 of referring our sensations to causes outside ourselves (perception), de- 

 pends 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 at- 

 tain 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. 



Judgments. 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 

 j'n reality a judgment based on many things besides the appearance of 



