CLASSIFICATION OF THE SENSES. 269 



external or rather the exterior senses, or those in which the sensa- 

 tions are projected to the exterior of the body, and which form, 

 therefore, the means through which we become acquainted with the 

 outside world. The exterior senses include sight, hearing, taste, 

 smell, pressure, and temperature (heat and cold). (2) The internal 

 or interior senses, or those in which the sensations are projected to 

 the interior of the body. It is through these senses that we acquire 

 a knowledge of the condition of our body and perhaps also a knowl- 

 edge of ourselves as an existence or organism distinct from the ex- 

 ternal world. Among the interior senses we must include pain, 

 muscle sense, the sensations from the semicircular canals and ves- 

 tibule of the internal ear, hunger, thirst, sexual sense, fatigue, and 

 in addition perhaps other less definite sensations from the visceral 

 organs. This line of demarcation, although it holds so well in 

 most cases, is not absolutely distinctive. The temperature sense, 

 for instance, is, so to speak, on the border line between the tw^o 

 groups; we may project this sensation either to the exterior or 

 to the interior according to circumstances. When the temperature 

 nerves are excited simultaneously with the pressure nerves, we 

 project the sensation to the exterior, to the stimulating body. If 

 the skin is touched by a hot or cold solid object we speak of the 

 object as being hot or cold. If, however, the same nerves are 

 stimulated by warm gases or even liquids under conditions that 

 do not involve the pressure sense we refer the change to ourselves, — 

 we are hot or cold, as the case may be. So also w^hen the skin is 

 heated by the blood the resulting sensation is projected to the 

 skin. It would seem that the habit of projection is acquired by 

 experience, and that those senses whose organs are habitually 

 affected by objects from without we learn to project to the object 

 giving rise to the stimulus. 



The Doctrine of Specific Nerve Energies. — The term specific 

 nerve energy we owe to Johannes MiiUer (1801-1858). The term 

 is in some respects unfortunate, as at present in the physical sci- 

 ences the word energy is used to designate certain specific properties 

 of matter. The phrase specific nerve energy in physiology, however, 

 is intended to designate the fact that each sensory unit arouses or 

 mediates its owti specific quality of sensation, the specific energy of 

 the optic apparatus being visual sensations, of the auditory apparatus 

 sound sensations, etc., and each sensory nerve or apparatus can 

 give no other than its own quality of sensation. Whether this 

 specificity in the reaction of each sensory nerve is due to some pecu- 

 liarity in the nerve itself or its peripheral end-organ, or to a pecu- 

 liarity of the part of the brain in which it terminates Miiller left 

 an open question, although he called attention to the fact that 

 the central ending is capable of giving its specific effect in con- 



