148 



VETERINARY PHYSIOLOGY 



The sphincter centre is reflexly called into action, and the 

 pupil contracted — 



1st. When strong light falls on the retina and stimu- 

 lates the optic nerve. In this way the retina is pro- 

 tected against over-stimulation. The two eyes act together 

 in this reflex ; if one be covered the pupils of both eyes 

 dilate. In the horse the light-reflex is sluggish. 



2nd. When the image upon the retina becomes blurred 

 as the object approaches the eye and the centre for the 

 ciKary muscle is called into play to produce accommoda- 

 tion. 



ID.N 

 2 D.N 



JD.N. 



Fig. 69.— Nerve Supply of the Intrinsic Muscles of the Eye (see text). 



3rd. The sphincter centre is also stimulated by 

 morphine and other drugs, and in the first stages of 

 asphyxia. In chloroform anaesthesia the pupil at first 

 responds to light, later, when the anaesthesia is full, it 

 is contracted. If the chloroform is pressed further, it 

 dilates and does not respond to light. This is a danger 

 signal. 



4th. In sleep the pupil is contracted. 



The centre for dilatation of the pupil is situated in the 

 medulla oblongata. Like the centre of the sphincter it may 

 be reflexly excited stimulation of ingoing nerves causing 



