14: SPECIAL SENSES. 



nating in organs calculated to receive the impressions of 

 smell, sight, hearing, and taste. 



The senses of olfaction, vision, audition, and gustation 

 belong to peculiar organs, provided with nerves of special 

 properties, which are not usually endowed with general sen- 

 sibility. These nerves have been omitted in our general 

 study of. the nervous system ; and the accessory organs to 

 which they are distributed are so important and intricate in 

 their structure as to demand extended description. 



The senses of touch, titillation, temperature, and pain 

 are all conveyed to the nerve-centres by what we have de- 

 scribed as ordinary sensory nerves ; the touch being perfected 

 in certain parts by peculiar arrangements of the terminal 

 nerve-fibres. Though it be possible that each one of these im- 

 pressions may be transmitted by special and distinct fibres, 

 this has not yet approached a positive demonstration. The so- 

 called muscular sense, by which we appreciate weight, resist- 

 ance, etc., undoubtedly depends, to a great extent, if not en- 

 tirely, upon the muscular nerves. In our study of the passage 

 of the nerve-fibres to the encephalon through the spinal cord, 

 it has been seen that most of the motor fibres decussate in 

 mass at the medulla oblongata, and that the sensory fibres de- 

 cussate throughout the entire length of the spinal axis. This 

 important anatomical and physiological fact enables us to 

 separate pretty clearly the muscular sense, so called, from 

 the various modifications of general sensibility just men- 

 tioned. Dr. Brown-Sequard has observed, in a number of 

 cases of disease of the cord, etc., in the human subject, 

 that the conductors of the impressions of touch, titillation, 

 pain, and temperature decussate in the cord, while the con- 

 ductors of the " muscular sense " if such a sense exist de- 

 cussate at the medulla oblongata. 1 



1 BROWN-SEQUARD, Recherches sur la transmission des impressions de tact, 

 de chatouillement, de douleur, de temperature, et de contraction (sens muscu- 

 laire) dans la moelle epinere. Journal de la physiologic, Paris, 1863, tome vi., 

 p. 125. 



