854 Or SENSATION, AND THE ORGANS OP THE SENSES. 



that nerve is usually subservient. Thus, congestion in the nerves of common 

 sensation gives rise to feelings of pain or uneasiness; but when occurring in the 

 retina and optic nerve it produces flashes of light ; and in the auditory nerve it 

 occasions "a noise in the ears." But further, the phenomena of subjective sen- 

 sation often originate in peculiar conditions of the Encephalon itself, and not 

 in the organs of sense or the nervous trunks; thus, in dreaming, we have 

 frequently very vivid pictures of external objects presented to our minds; and 

 we sometimes distinctly hear voices and musical tones, or have perceptions 

 (though this is less common) of tastes and odors. The phenomena of spectral 

 illusions are very nearly connected with those of dreaming; both may be in 

 some degree influenced by external causes, acting upon the organs of sensation, 

 which are misinterpreted (as it were) by the mind, owing to its state of imper- 

 fect operation; but both also may entirely originate in the central organs. 

 There seems to be no difference, in the feelings of the individual, between the 

 sensations thus originating, and those which are produced in the usual manner; 

 for we find that, unless otherwise convinced by their own reason, persons who 

 witness spectral illusions believe as firmly in the reality of the objects that come 

 before their minds, as if the images of those objects were actually formed on 

 their retinae. This is another proof, if any were wanting, that the organ of 

 sense, and the nerve belonging to it, are but the instruments by which certain 

 changes are produced in the Sensorium; by which changes, and not by the 

 immediate impressions of the objects, our Consciousness is really affected. 

 There is yet another mode, however, in which subjective sensations may be ex- 

 cited; namely, by sensations originating in objective impressions on other parts. 

 Thus the irritation of a calculus in the bladder gives rise to pain at the end of 

 the penis; disease of the hip-joint is often first indicated by pain in the knee; 

 irritation of the ovary will cause pain under the mamma ; various disorders of 

 the liver occasion pain under the left scapula; attention is often drawn to 

 diseases of the heart by shooting pains along the arms; stimulation of the nip- 

 ple, whether in the male or female, gives rise to peculiar sensations referred to 

 the genital organs; the sudden introduction of ice into the stomach will cause 

 intense pain in the supraorbital region, and the same pain is frequently occa- 

 sioned by the presence of acid in the stomach, and may be very quickly relieved 

 by its neutralization with an alkali. It will be seen that in most of these cases 

 it is impossible to refer the sensations to any direct nervous connection with the 

 parts on which the impressions are made; and they can scarcely be otherwise 

 accounted for than by supposing that these impressions produce sensorial 

 changes, which are referred to other parts, in virtue of some central track of 

 communication with them, analogous to that through which reflex movements 

 are excited. There are circumstances, indeed, which seem to render it not 

 improbable that, just as the impression brought by the afferent nerves to the 

 central organs, excites a reflex movement by disturbing the polarity of a motor 

 nerve, it may excite a "reflex sensation" by disturbing the polarity of a sensory 

 nerve. Certain it is that, after the long continuance of some of these reflex 

 sensations, the organs to which they are referred themselves become diseased, 

 although previously quite healthy; thus, pain in the testicle is frequently in- 

 duced by irritation having its seat in the lower part of the spine, but, if this 

 continue, some morbid affection of the testicle itself is likely to supervene ; and 

 Sir B. Brodie 1 has recorded several cases, in which " nervous" pains in various 

 parts, apparently of a purely subjective character, have been followed by pain 

 and swelling of the integuments. These phenomena are perhaps due to that 

 habitual direction of the consciousness to the part which is prompted by the 

 habitual sensation; this condition, as we shall see hereafter (CHAP, xvin.), 



1 "On Local Nervous Affections," 1837. 



