392 COMPARATIVE ZOOLOGY OF CHORDATES 



By its contraction, the distance between the lens and the retina 

 is increased, and the eye can then focus near objects. 



In all the cases so far mentioned, the lens is a rigid body 

 with a fixed and definite focal length, and which has to be 

 moved bodily in order to accommodate the eye. In the 

 remaining vertebrates, the lens is elastic and capable of varying 

 its convexity and focal length. In reptiles, accommodation for 

 near vision is brought about by contraction of the circular 

 muscle of the iris, which has as its effect the increase in con- 

 vexity of the lens, which thus tends to become spherical. 

 In the birds, there is in addition a striated muscle called 

 Crampton's muscle, contraction of which decreases the diameter 

 of the eyeball in the neighbourhood of the junction between 

 the cornea and the sclerotic. This causes the surface of the 

 cornea to become more convex, and assists the lens to bring 

 rays of light from near objects to a focus on the retina. 



The method of accommodation in the mammals differs 

 from that in other vertebrates. The lens is suspended by the 

 suspensory ligament, which is kept tense by the elasticity of 

 the lens trying to revert to the spherical shape. The suspensory 

 ligament is attached to the ciliary process. The ciliary muscle 

 is attached to the cornea in front and to the choroid behind, so 

 that when it contracts, the choroid and ciliary process are 

 brought forwards. This forwards movement of the ciliary 

 process reduces the tension on the suspensory ligament, and 

 the lens is allowed to become more spherical, which increases 

 its refractive power and enables it to accommodate the eye to 

 near objects. The change in focal length of the lens is there- 

 fore only indirectly due to the action of the ciliary muscle. 



In some vertebrates, and especially those of nocturnal 

 habits, the eyes do not accommodate for distance at all, which 

 fact does not prevent them from enjoying good sight, as does 

 the owl. In daylight, the pupil may be so contracted as to 

 simulate a " pinhole " camera, in which accommodation is 

 unnecessary. 



In mammals the ciliary muscle is contracted by impulses 

 passing in fibres of the parasympathetic system through the 

 oculomotor nerve and the ciliary ganglion. Other fibres 



