Chap. XX] INTERNAL AND EXTERNAL SENSES 409 



ous obscure sensations which proceed from the viscera and give us 

 the feehng of well-being or the reverse, also the desire for defecation 

 or urination. 



External or exterior senses are those in which the sensations are 

 projected to the exterior of the body. They form the means by 

 which we become acquainted with the outside world. They 

 include pressure and temperature sense, taste, smell, hearing, and 

 sight. Even this classification is not absolutely distinctive, as 

 some sensations may be projected either to the interior or exterior 

 of the body. Temperature and pain are examples of this class. 



Hunger. — Hunger occurs normally at a certain time after meals 

 and is usually projected to the region of the stomach. It is pre- 

 sumably due to contractions of the empty stomach, which stimu- 

 late the nerves distributed to the mucous membrane. In abnormal 

 conditions the stomach need not be empty, for it must be remem- 

 bered that physiologically food is not considered as being inside the 

 body until it ha.s been digested and absorbed. Thus a diabetic 

 may feel very hungry, although he has within a short time partaken 

 of a huge meal. 



Thirst. — This sensation is projected to the pharynx. We 

 know very little about the nervous mechanism involved, but it is 

 thought that when the water content in the tissues falls below a 

 certain amount the sensory nerve fibres in the pharynx are stimu- 

 lated and' produce the sensation of thirst. The sense of thirst 

 is more imperative than that of hunger ; a person can live several 

 days without food, provided he has water, but if this latter is 

 denied, he will soon die of exhaustion. 



Pain. — The sensation of pain is thought by some authorities 

 to be due to stimulation of special nerves that give rise to the 

 sense of pain. 



Other authorities question the existence of special nerves for 

 thivs sense, and think it is due to overstimulation of any of the 

 sensory nerves. For instance, extreme pressure or extremes of 

 temperature cause overstimulation of the nerves of pressure or 

 temperature, and the result is painful. 



Muscular sense. — The end organs of the muscular sense are 

 situated in the tendons and between the fibres of the muscles. 

 They convey to us the sense of resistance in the muscles when 

 we attempt to lift anything. This is the muscular sense. 



