THE SENSES 535 



contraction of the dilator muscle and inhibition of the circular 

 muscle. 



The nerve supply to the two muscles of the iris is not the same ; 

 the circular fibres are supplied with motor power through the 

 third cranial nerve, whilst the dilator muscle is supplied by 

 the sympathetic. The latter fibres arise in the mid-brain, run 

 down the cord, and terminate in cells in the lower cervical 

 region. Here fresh neurones arise which emerge from the spinal 

 cord at the eighth cervical and first three thoracic spinal nerves, 

 from a part known as the cilio-spinal centre ; they travel up the 

 neck in the cervical sympathetic, connect with the superior 

 cervical ganglion, thence to the Gasserian ganglion, and by 

 means of the long ciliary nerves in the ophthalmic branch of the 

 fifth nerve they reach the iris. The fibres of the third nerve 

 connect with the ciliary ganglion, and by means of the short 

 ciliary nerves (post-ganglionic fibres, now autonomic) they reach 

 the iris. If the third nerve be divided, the radiating muscular 

 fibres of the iris contract under the unbalanced action of the 

 sympathetic, and thus dilate the pupil ; if the sympathetic be 

 divided, the pupil contracts under the unbalanced action of the 

 sphincter fibres. Under ordinary conditions both constrictor 

 and dilator muscles are receiving impulses, which neutralise each 

 other, so that the iris is readily responsive to any excess of im- 

 pulses which disturb the balance. This act is a true reflex, being 

 carried out through the afferent fibres of the optic nerve, a centre 

 in the brain probably situated in the floor of the aqueduct of 

 Sylvius, and an efferent path furnished by the third pair of 

 nerves. Stimulation of the retina by light is the natural method 

 by which alterations in the size of the pupil are brought about ; 

 in a brilliant light the pupil contracts, in a low light it dilates. 

 In the horse this is not strictly true ; in direct sunlight the pupil 

 of this animal is a mere narrow chink, but in ordinary diffused 

 daylight it barely responds, or if it does contract it is so little as 

 not materially to reduce the size of the pupil. Even when light 

 is concentrated on the eye, either by means of a mirror or a lens, 

 the iris practically remains unchanged. Under the influence of 

 artificial light it actually dilates. In all herbivora the pupil is 

 relatively sluggish in response to the stimulus of light. The con- 

 traction is greater in the exposed than in the opposite eye. 

 Harris,* indeed, regards this fact as an indication of a low stan- 

 dard of visual acuity in the herbivora. In the cat the reaction of 

 the pupil is brisk, and sharper than in the case of the dog. The 

 above observer considers that want of visual acuity is com- 

 pensated in the herbivora by greater keenness of hearing and 



* ' Binocular and Stereoscopic Vision in Man and Other Vertebrates.* 

 W. Harris, M.D., M.R.C.P. ; Brain, part iv., 1904. 



