Afferent 



OR 



Sensory. 



Chap. XIX.] ORGANS OF SPECIAL SENSE. 223 



tarily or reflexly, caused to contract. All peripheral nerve 

 fibres may thus be classified by the way in which they termi- 

 nate, or, what is the same thing, by their physiological function. 

 The following is such a classification : — 



_ \ Voluntary (endintr in the voluntary musclesV 



Efferent k i / / ^ • ^ j j-i ^ j- 



I Involuntary (e.g. vaso-constrictor and vaso-dilator ; cardio- 



, , j accelerator and cardio-inhibitory, etc.). 



[^ Secretory (ending in gland cells). 



r Reflex sensory (unaccompanied by the phenomena of conscious- 

 ness). 

 Special sensory (accompanied by conscious sensation), viz. : — 

 Pressure. Pain. Hearing. 



Heat. Muscle-sense. Equilibrium. 



Cold. Taste. Vision. 



In the preceding chapters ^ attention has been called to 

 different varieties of efferent nerves, and to the fact that any 

 of these nerves might be stimulated reflexly through appropriate 

 afferent {i.e. reflex sensory) nerves. We have now to consider 

 those afferent fibres, the special sensory, which are concerned 

 with the special senses, and in connection therewith to study 

 the structures in which these nerves terminate, and which are 

 called the organs of special sense. 



Touch or pressure. — The special organs of the sense of touch 

 (Fig. 128) are distributed over the entire surface of the body, 

 being more or less numerous in all parts of the true skin. 

 Stimulation of these organs produces a sensation of touch, and 

 we distinguish not only differences in the intensity of the stimu- 

 lus, but also the locality in which the stimulus is applied. The 

 sensations produced by the stimulation of the touch endings in 

 different parts of the body resemble each other, but are not iden- 

 tical. We have learned by experience to associate these differ- 

 ences (which are called the "local signs ") with the locality in 

 which the end organ stimulated is situated. Thus if the hand be 

 stimulated we have three perceptions in consciousness : first, 

 that we have been touched; secondly, we are conscious of the 

 degree of pressure, i.e. of the intensity of the stimulus; and 

 thirdly, we are aware of tlie fact that it is the hand which has 

 been touched. 



1 Nerves to Voluntary Muscles, page 72 ; Vaso-constrictor Nerves, page 137 ; 

 Vaso-dilator Nerves, page 137 ; Cardio-accelerator, page 110 ; Cardio-inhibitory 

 Nerves, page 110 ; Secretory Nerves, page 166. 



