242 HUMAN PHYSIOLOGY 



against the will. Forced movements in animals are, for example, 

 the circus movements, clock-hand movements, rolling movements. 

 In normal individuals forced movements may be observed during 

 dizziness caused by rotation. 



2. Centres for the movements of the eyes. All the 

 centres for the movements of the eyes lie in the gray matter 

 forming the floor of the aqueduct of Silvius and the fourth 

 ventricle (except the centre for closing the eyelid and the 

 pupil reflex, see below and page 233). 



(a) The centres for tJie coordinated movements of both 

 eyes. Concerning the functions of the individual centres, 

 see page 280. Reflexes brought about by means of these 

 centres are: 



1. Involuntary movements of the eye (afferent impulse 

 travelling through the optic nerve) by which the eye follows 

 a moving object or by which the glance is thrown upon a 

 luminous object. 



2. Reflexes which are called forth by the sense organs 

 for perceiving the position and movements of the head 

 (semicircular canals of the ear, the centripetal nerve being 

 the auditory). In this group belong the compensatory 

 movements of the eyes which are involuntarily made when 

 the head is moved, in order that the line of vision may 

 remain on fixed objects. 



Forced movements of the eye which occur in diseases of these 

 centres and their paths are called nystigmus. 



(b) Centre for the common inner-nation of accommodation, 

 convergence, and contraction of the pupil. This is voluntarily 

 stimulated during near vision. 



(c) Centre for the closing of tJie eyelids. This is volun- 

 tarily or reflexly stimulated. Reflex stimulation occurs 

 when the cornea or conjunctiva is touched (centripetal 

 nerve is the first branch of the trigeminus), or by stimulation 

 of the optic (blinking). The centrifugal nerve is the facial 

 which innervates the palpebrarurn orbicularis. The centre 

 is situated in the medulla oblongata. 



Centres for the regulation of body temperature. It is supposed 

 by some authors that at the boundary between the medulla and the 



