DIOPTRIC MECHANISMS OF THE EYEBALL 611 



seems to be connected with the rapid accommodation that is necessary when a 

 bird swoops down towards the ground to pick up some food insect. Moreover, 

 since binocular vision is not present in many birds, and convergence of the 

 optic axes must be minimal, it is probable that the contractions of Crampton's 

 muscle play a great part in guiding the movements of the bird, and especially 

 in aiding it to judge distances. In ourselves such judgment is very faulty 

 without the co-operation of the two eyes. 



In amphibia and snakes, which at rest are also focused for distance, active 

 accommodation for near objects is effected, not by change in curvature of the 

 lens, but by an increase in the distance between the lens and the retina. In 

 amphibia the ciliary muscle, which lies between the root of the iris, the sclerotic 

 and choroid, causes a rise of pressure in the vitreous cavity, and the lens, being 

 the most movable part of the boundary wall of this cavity, is pushed forwards 

 towards the cornea. The aqueous humour, which is displaced by this forward 

 movement of the lens, finds a place in the lateral angle of the eye, which is 

 increased in depth by the pull of the muscle fibres. 



In snakes the same action is effected by a muscle, often cross-striated, which 



R A 



FIG. 275. Accommodation in eye of sepia. (BEER.) 

 R, at rest ; A, during accommodation (for distance). 



is situated in the root of the iris. In both these cases the movement of accom- 

 modation is unaffected by opening the aqueous cavity, whereas in mammals 

 it is at once rendered impossible if the aqueous cavity be laid open. 



Most of the teleostean fishes are short-sighted, i.e. at rest they are focused 

 for near objects. Active accommodation of these animals diminishes the 

 refractive power of the eye, so that accommodation occurs for distant objects. 

 In the fish's eye there are no ciliary processes, ciliary muscle, zonule of Zinn, 

 or spaces of Fontana, such as are found in the higher vertebrata. The iris 

 only approaches the margin of the lens, and does not shut out its peripheral 

 rays. The lens, which is spherical (Fig. 274), is hung up by means of a flat 

 band attached to its upper pole. This is known as the ' suspensory ligament,' 

 but is quite different in structure and mechanism from the suspensory ligament 

 or zonule of Zinn of the vertebrate eye. From the lower and inner pole of 

 the lens a dark pigmented structure passes backwards ; this was described 

 by its first discoverer as the campanula, but since it is muscular in character 

 is better named the 'retractor lentis.' On stimulation this muscle pulls the 

 lens backwards, and so lessens the distance between it and the retina, in this 

 way accommodating the eye for distance. 



The eye of the cephalopod mollusc, such as sepia, is also short-sighted, and 

 active accommodation, as in the fish's eye, is accommodation for distance. 

 The mechanism is, however, quite different. The globe of the cephalopod's 

 eye has the shape shown in the diagram (Fig. 275). The most resistant part 

 of the globe is formed by a strong ring of cartilage which passes round the 

 equator of the eye. The rest of the sclerotic is formed of delicate membrane, 

 which is thinnest in the ring just behind the cartilaginous ring, In the anterior 



