754 



SPECIAL SENSES. 



nature of the sensation is due to the unusual character of the impression, and it does not 

 involve the action of special nerve-fibres as conductors. 



Appreciation of Temperature. It is not known that the sense of temperature, either 

 of the surrounding medium or of bodies applied to different parts of the skin, is appreci- 

 ated through any nerves other than those of general sensibility, or that there is any special 

 arrangement of the terminations of certain of the nerves connected with this sense. As 

 regards the general temperature, the sense is relative aud is much modified by habit. 

 This statement needs no explanation. As is well known, what is cold for an inhabitant 

 of the torrid zone would be warm for one accustomed to an excessively cold climate. 

 Habitual exposure also modifies the sense of temperature. Many persons not in the habit 

 of dressing warmly suffer but little in extremely cold weather. Those who habitually 

 expose the hands, or even the feet, to cold, render these parts quite insensible to tempera- 

 ture ; and the same is true of those who often expose the hands, face, etc., to heat. 



The variations in the sensibility of different parts of the surface to temperature depend, 

 as we have just indicated, to a great extent upon habit, exposure, etc., but also upon 

 special properties of the parts themselves. The differences, however, are not so marked 

 as to be of any great importance, and the experiments made upon this point are simply 

 curious. It is remarkable, however, to note the exquisite sensibility to variations in tem- 

 perature sometimes presented by those who are deprived of other senses. The example 

 is quoted by DungHson, of Dr. Saunderson, formerly Professor of Mathematics at Cam- 

 bridge, England, who, " when some of his pupils were engaged in taking the altitude of 

 the sun, could tell, by the slight modification in the temperature of the air, when very 

 light clouds were passing over the sun's disk." 



The experiments of Weber show conclusively that the skin is the main organ for the 

 appreciation of temperature, if we except the mouth, palate, vagina, and rectum, by which 

 the difference between warm and cold substances is readily distinguished. In several 

 instances in which large portions of the skin were destroyed by burns and other injuries, 

 experiments have been made by applying spatulas of different temperatures. At one time 

 a spatula plunged in water at from 48 to 55 Fahr. was applied to a denuded surface, 

 and again, a spatula at from 113 to 122 Fahr. When the patient was requested to tell 

 which was the warmer, the answers were as frequently incorrect as they were correct ; 

 but the discrimination was easy and certain when the applications were made to the sur- 

 rounding healthy skin. When applications at a higher temperature were made to the 

 denuded part, the patient suffered only pain. 



The venereal sense, which we shall not attempt to describe, is unlike any other sensa- 

 tion, and is general, as well as referable to the organs of generation. In this connection, 

 however, it is interesting to note that the tactile sensibility of the palmar surface of the 

 third phalanx of the fingers, measured by the aBsthesioraeter, compared with the sensi- 

 bility of the penis, is as 0-802 to 0-034, or between twenty-three and twenty-four times 

 greater. 



Olfactory Nerves. 



The nerves directly connected with the senses of olfaction, vision, and audition, are 

 but slightly if at all endowed with general sensibility. As regards the olfactory nerves, 

 the parts to which they are distributed are so fully supplied with branches from the fifth, 

 that it is difficult to determine the fact of their sensibility or insensibility to ordinary 

 impressions. The olfactory nerves, however, are distributed to the mucous membrane 

 of that portion of the nasal cavity endowed with the special sense of smell. Before 

 taking up their physiological anatomy, we shall describe briefly the parts to which the 

 olfactory sense is probably confined. 



Nasal Fosses. The two irregularly-shaped cavities in the middle of the face, opening 

 in front by .the anterior nares and connected with the pharynx by the posterior nares, 



