VISION 983 



a distinct effect. In two cavities separated by a slack membrane no 

 differences of pressure would be expected. 



Anderson Stuart lays stress upon the function of those fibres of the 

 suspensory ligament which are attached to the vitreous body, and are 

 put under tension by the contraction of the ciliary muscle, in anchoring 

 the lens during strong accommodation. He believes that the liquid 

 contents of the hyaloid canal move from its anterior to its posterior end 

 in accommodation, and in the opposite direction when accommodation 

 is relaxed, and that this movement tends to prevent strains in the 

 vitreous. 



In cephalopods and fishes, which are normally short-sighted, accom- 

 modation for objects at a distance is effected by a movement of the lens 

 towards the retina. In the fish's eye this is accomplished by the con- 

 traction of a special muscle, the retractor lentis. In amphibia and most 

 snakes the lens is moved towards the cornea and away from the retina 

 by changes of intra-ocular pressure (Beer). 



Innervation of the Ciliary Muscle and the Muscles of the Iris. The 

 ciliary muscle and the sphincter pupillae are supplied by autonomic 

 fibres (p. 964), reaching them through the short ciliary nerves arising 

 from the ciliary ganglion (Fig. 403). The preganglionic fibres take- 

 origin from cells in the anterior part of the oculo-motor nucleus in the 

 mid-brain. Passing to the orbit in the third nerve, they reach the 

 ciliary ganglion, and end there by forming synapses with some of its 

 cells. The axons of these cells continue the path as post-ganglionic 

 fibres in the short ciliary nerves. The dilator pupillae is supplied by the 

 long ciliary nerves coming from the ophthalmic branch of the fifth 

 nerve. 



The preganglionic dilator fibres pass out by the anterior roots of the 

 first three thoracic nerves (dog, cat, rabbit), accompanied by vaso- 

 constrictor fibres for the iris. Reaching the sympathetic chain through 

 the corresponding rami communicantes, they traverse the first thoracic 

 ganglion, the annulus of Vieussens, the inferior cervical ganglion, and 

 the cervical sympathetic. They end by arborizing around some of the 

 cells of the superior cervical ganglion, whose axons eventually arrive at 

 the Gasserian ganglion, and running along the ophthalmic division of 

 the trigeminal to the eye, reach the iris by its long ciliary branches. 



The exact origin of the dilator path in the brain has not been defi- 

 nitely settled. Some place it in the mid-brain, others in the bulb. 

 There must be at least one neuron on the path central to the spinal 

 neuron whose axon emerges from the cord as a preganglionic fibre. 

 The lower cervical and upper thoracic portion of the spinal cord has 

 received the name of the cilio-spinal region from its relation to the 

 pupillo-dilator fibres. It must not be looked upon as a centre in any 

 proper sense of the term, but rather as the pathway by which these 

 fibres pass down from the bulb, and where they may accordingly be 

 tapped by stimulation. 



Stimulation of certain areas on the cortex of the frontal lobe of the 

 cerebrum (p. 962) causes slight dilatation of the pupil even after the 

 sympathetic has been divided. This is due to inhibition of the pupil lo- 

 constrictor fibres in the third nerve. 



Changes in the Pupil during Accommodation. It has been already 

 mentioned that along with the alteration in the curvature of the 

 lens a change in the diameter of the pupil takes place in accommo- 

 dation. When a distant object is looked at, the pupil becomes 

 larger ; when a near object is looked at, it becomes smaller. Narrow- 



