THE SENSES. 527 



THE SENSES. 



It is by the senses that the individual is brought into relation 

 with the world outside him. The senses are five in number : (1) 

 General sensibility; (2) Smell; (3) Taste; (4) Sight; and (5) 

 Hearing. 



General Sensibility. This kind of sensibility is so called 

 because it is generally distributed over the entire body in the 

 skin, and in those parts of mucous membrane adjacent to the skin. 

 It is composed of a variety of sensations which are excited by a 

 variety of stimuli, but it is still an unsettled question whether the 

 nerves which conduct the impulses that excite these sensations are 

 in all instances the ordinary sensory nerves of the skin, or whether 

 they are special nerves, each one conducting only its own special 

 stimulus. In treating of the subdivisions of general sensibility 

 this question will again be referred to. 



Sense of Touch. The sense of touch, or tactile sensibility, de- 

 pends upon the existence of nerves and nerve-endings (p. 64) in 

 the skin and other portions of the body in which the function 

 exists. It gives knowledge of such qualities as hardness or soft- 

 ness, roughness or smoothness, sharpness or dulness, etc. ; by it 

 we become acquainted with the shape and consistency of objects, 

 and are made aware of the presence or absence of irritating quali- 

 ties in certain substances. The pungent vapors of some gases 

 excite in the nose the ultimate fibers of distribution of the fifth 

 pair of nerves, and not those of the first pair, and it is incorrect 

 to describe this sensation as a smell. It is as truly a tactile sensa- 

 tion as when a sharp-pointed instrument is brought in contact 

 with the skin. The same is true of pungent liquids applied to the 

 tongue, which are commonly, but erroneously, said to be tasted. 



The difference in the tactile sensibility of different portions of 

 the body is shown in the following table : 



TABLE or VARIATIONS IN THE TACTILE SENSIBILITY or DIFFERENT 



PARTS. 



The measurement indicates the least distance at which the two points of a 

 pair of compasses can be separately distinguished (E. H. Weber). 



Tip of tongue 1 mm. 



Palmar surface of third phalanx of forefinger ..... 2 



Palmar surface of second phalanges of fingers 4 



Red surface of under lip 4 



Tip of the nose ' 6 



Middle of dorsum of tongue 8 



Palm of hand 10 



Center of hard palate 12 



Dorsal surface of first phalanges of fingers 14 



Back of hand 25 



Dorsum of foot near toes 37 



Gluteal region 37 



Sacral region 37 



