548 



PHYSIOLOGY 



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

 modation 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 is 

 situated in the root of the iris. In both these cases the movement of accommodation 

 is unaffected by opening the aqueous cavity, whereas in mammals it is at once rendere d 

 impossible if the aqueous cavity be laid open. 



FIG. 275. Accommodation in a bird's eye. (BEER.) 

 B, rest ; A, accommodation for near objects. 



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

 objects. Active accommodation in 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. 276), is 

 hung up by means of a flat band attached to its uppei pole. This is known as the 

 * suspensory ligament,' but is quite different in structure and mechanism from the sus- 

 pensory 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. 277). 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 cartilagin- 

 ous ring. In the anterior wall of the eye is a strong muscular ring, composed of 

 meridional fibres, which run from the cartilaginous ring to be inserted into the ciliary 

 processes or corpus ciliare, which is closely attached to the equator of the lens. 



When this muscle contracts it pulls back the whole anterior wall of the eye together 

 with the lens, approximating it to the retina. This movement is of necessity accom- 

 panied by a rise of ocular pressure, but room for the displaced fluid is found by a bulging 

 of the walls of the eyeball at their thinnest part, i.e. just behind the cartilaginous 



