216 PRINCIPLES OF PHYSIOLOGY 



sense of equilibrium, and of the position 

 of the head in space. These external senses 

 have always a special mechanism, namely 

 (a) an end organ adapted for the reception of 

 a specific kind of stimulus ; (b) a nerve of 

 special sense ; and (c) an internal receptive 

 organ in the brain, which may act without 

 consciousness in reflexes, or with conscious 

 perception in the cerebrum. As an example 

 take the sense of vision. The normal stimulus 

 is light, the end organ is the retina, the nerve 

 is the optic nerve, the recipient centre is in 

 the first instance the corpora quadrigemina, 

 or optic lobes (as they are termed, for example, 

 in birds), and the centre of sensations of light 

 and colour are in the cerebrum, more especially 

 in a special area of grey matter. 



126. The senses may be classified thus : 

 (a) Those in which the stimulus is movement 

 or pressures, namely touch, hearing, the 

 muscular sense, and the sense of equilibrium ; 

 and (b) those in which the stimulus is more 

 of a molecular character, implying chemical 

 action, namely vision, taste, and smell. In 

 the outer world, according to the conceptions 



