GENERAL CHARACTERS OF THE SPECIAL SENSES. 653 



the general description of the nervous system, as well as the accessory organs 

 to which they are distributed. 



The senses of touch, temperature and pain are all conveyed to the nerve- 

 centres by what have been described as sensory nerves, the touch being per- 

 fected in certain parts by peculiar arrangements of the terminal nerve-fibres. 

 Although it is possible that each one of these impressions is transmitted by 

 special and distinct fibres, this is not yet a matter of positive demonstration. 

 The so-called muscular sense, by which weight, resistance etc., are appreci- 

 ated, undoubtedly depends to a great extent upon the muscular nerves. What 

 are generally recognized as sensory impressions have been thus subdivided. 

 These subdivisions are sufficiently distinct, as far as the character of the 

 sensations themselves are concerned, but as regards their paths of conduc- 

 tion, as before intimated, exact and positive data are wanting. It is impossi- 

 ble to study with advantage the different varieties of ordinary sensation in 

 the lower animals, for evident reasons ; and physiologists rely mainly upon 

 observations on the human subject, in the form of experiments and of patho- 

 logical phenomena. 



There are two ways of regarding the different varieties of general sensa- 

 tion : One is to look at each as a peculiar impression conveyed by special 

 nerve-fibres, and the other is to regard the nerves of general sensibility as 

 capable of conducting impressions of different kinds. It has never been 

 assumed that special fibres for each variety of sensation have been demon- 

 strated, and it is possible only to reason as to this from what is actually 

 known of the general properties of sensory nerves. 



The general sensory nerves are sufficiently distinct in their properties 

 from the true nerves of special sense. The latter convey peculiar impressions 

 only, such as those of sight, hearing, smell and taste. The former, when 

 strongly stimulated or irritated, always convey impressions of pain. Separat- 

 ing, then, all other senses, except the venereal sense, from the true special 

 senses, it is proper to inquire whether it be reasonable or necessary to assume 

 that any of the varieties of general sensation require special nerves for their 

 conduction. 



It is well known that a relatively powerful stimulation of a sensory nerve 

 or of sensitive parts is necessary to the production of a painful impression ; 

 and it is also well known that very painful impressions overpower impressions 

 of touch, weight, pressure, temperature and the so-called muscular sense. In 

 cases of disease, it is sometimes observed that tactile sensibility is retained in 

 parts that are insensible as regards pain. It is possible that sensory nerve- 

 fibres may become so altered in their properties as to be incapable of con- 

 ducting painful impressions, while they still conduct sensations that are 

 appreciated only as impressions of contact. This is observed in certain cases 

 of artificial anaesthesia. In hyperaesthesia, or exaggerated sensibility to pain- 

 ful impressions, the tactile sense is necessarily overpowered in a greater or 

 less degree. Impressions made on a sensory nerve in its course are always 

 appreciated as painful, and the pain is referred to the terminal distribution 

 of the nerve, this being a law of sensory perception. There is no sense of 



43 



