548 HANDBOOK .OF PHYSIOLOGY. 



The exciting cause may be (in the vast majority of cases is), some ob- 

 ject of the external world (objective sensation); or the condition of the 

 sensorium may be due to some excitement within the brain, in which 

 case the sensation 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 external 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. 



Cases 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 sen- 

 sations of light and luminous flashes; in the auditory nerve, the sensa- 

 tion 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 

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

 "tinnitus aurium; " and in the common sensory nerves, the sensation of 

 creeping over the surface. So, also, among external causes, the stimu- 

 lus 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 jar- 

 ring 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. Thus, Ma- 

 gendie observed that when the olfactory nerves, laid bare in a dog, were 

 pricked, no signs of pain were manifested; and other experiments of his 

 seem to show that both the retina and optic nerve are insusceptible of 

 pain. Further, the optic nerve is insusceptible to the stimulus of light 

 when severed from its connection with the retina which alone is adapted 

 to receive luminous impressions. 



Sensations and Perceptions. The habit of constantly referring 

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

 fications which external objects produce in our sensations, as properties 



