908 A MANUAL OF PHYSIOLOGY 



muscle has been very strongly contracted by eserine the lens can be 

 observed to move about with each slight movement of the eye. The 

 suspensory ligament must therefore be slackened by the contraction 

 of the ciliary muscle. When atropine is applied the movability of 

 the lens soon disappears, owing to paralysis of the ciliary muscle. 

 These facts were first established in patients after iridectomy, but 

 have also been demonstrated in the normal eye. Even under the 

 influence of gravity alone, without any movements of the eye, the 

 lens sinks about to mm. in strong accommodation. An 

 additional proof that the suspensory ligament is perfectly slack 

 during accommodation is derived from the result of simultaneous 

 measurements in animals of the pressure in the anterior chamber 

 and in the vitreous. Even in strong accommodation no alteration 

 occurs, although even slight contact with the outer surface of the 

 eyeball or contraction of the external eye muscles causes 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, 

 accommodation 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 contraction of a special muscle, the retractor lentis. In am- 

 phibia 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 pupillse are supplied by autonomic 

 fibres (p. 883), reaching them through the short ciliary nerves arising 

 from the ciliary ganglion (Fig. 388). 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 pupillse 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 fibre? 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 

 definitely 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 



