160 HAND-BOOK OF PHYSIOLOGY. 



the external world (objective sensation}] or the condition of the sensorium 

 may be due to some excitement within the brain, in which case the sen- 

 sation is termed subjective. The mind habitually refers sensations to 

 external causes; and hence, whenever they are subjective (due to causes 

 within the brain), we can hardly divest ourselves of the idea of an ex- 

 ternal cause, and an illusion is the result. 



Illusions. Numberless examples of such illusions might be quoted. 

 As familiar cases may be mentioned, humming and buzzing in the ears 

 caused by some irritation of the auditory nerve or centre, and even musi- 

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

 optical illusions: persons and other objects are described as being seen, 

 although not present. Such illusions are most strikingly exemplified in 

 cases of delirium tremens or other forms of delirium, in which cats, rats, 

 creeping loathsome forms, etc., are described by the patient as seen with 

 great vividness. 



Causes of Illusions. One uniform internal cause, which may act 

 on all the nerves of the senses in the same manner, is the accumulation 

 of blood in their capillary vessels, as in congestion and inflammation. 

 This one cause excites in the retina, while the eyes are closed, the sensa- 

 tions 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 

 auriunr'; and in the common sensory nerves, the sensation 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; in the tongue, a saline or acid taste; and in the 

 other parts of the body, a perception of peculiar jarring or of the mechan- 

 ical impression, or a shock like it. 



Sensations and Perceptions. The habit of constantly referring 

 our sensations to external causes, leads us to interpret the various modifi- 

 cations 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 (percep- 

 tion), 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 



