342 SPECIAL PHYSIOLOGY. 



the sensations proper to the muscular system have been removed, and 

 treated of as belonging to a special sense, under the name of the sixth 

 sense. The researches of Schiffand Brown-Se'quard (page 271), have, 

 moreover, led those physiologists to the belief, that special paths of 

 conduction exist in the spinal cord, for the sensations of touch, temper- 

 ature, and pain ; so that perhaps the true tactile sense should be dis- 

 tinguished from common sensation, to which pain may be referred, and 

 the sense of temperature from both. 



Special Sensation. 



The causes of the variety of sensations, whether common or special, 

 are very obscure. Common sensation is the basis of the special sen- 

 sations, or the fundamental type of sensibility. Touch is plainly a 

 modification of it, and, in this general view, even taste, smell, hear- 

 ing, and sight, are but special adaptations of a common sensory fac- 

 ulty. The organs of the special senses, even the nose, the ear, and 

 the eye, are formed out of inverted portions of the common surface 

 of the embryo, which, for the development of those specially con- 

 structed and complex organs, undergo peculiar metamorphoses, to 

 adapt each for the reception and translation of its proper external 

 stimulus, so that this may excite the energy of the nerve and nervous 

 centres. Besides the special recipient apparatus, however, there are 

 special internuncial nerves, and special sensorial nervous centres. 

 Moreover, it is supposed that these nerves have special endowments to 

 suit them for their respective offices, and that the nerve centres possess 

 distinct physiological properties or modes of reaction under external 

 stimulation. 



The special susceptibility of each sensorial centre, whether of that 

 of sight, hearing, smell, or taste, is said to be proved by the fact, that 

 internal changes in these centres, may produce corresponding sensa- 

 tions, independently of the co-operation of either the special nerves 

 or recipient apparatus. . In cases of amaurosis, or loss of visual power 

 in the retina or nervous expansion of the eyeball, luminous spectra 

 have been excited by internal causes ; and galvanism, applied to a 

 person deprived of one eye, has been known to cause luminous impres- 

 sions on the blind side. But to these facts, it may be objected, that, 

 in the first case, the recipient organ was still connected with the brain, 

 and might not have been altogether disorganized ; and in the second 

 case, that the luminous impressions might have been erroneously re- 

 ferred to the blind side, or have been the result of a recurrent action 

 of the sound eye. A common internal stimulus, such as an excess of 

 blood in the capillaries of the nervous centres, is frequently known to 

 produce flashes of light, noises, or odors, according to the sensorial 

 centre affected ; but in such cases, it must be remembered that these 

 parts are still in connection with their respective recipient organs. 

 This is also the case, when so-called ocular spectra are seen, some of 

 which are so definite in shape, detail, and color, as to resemble external 

 objects, or persons, familiar to the subject of such spectral illusions, 

 and, indeed, so defined, that we cannot conceive of their production 



