504 



PHYSIOLOGY 



In man, as we shall see later, this is accomplished by adjusting the power of the lens. 

 In fish the eye, which is normally focussed for near objects, is caused to focus objects 

 at a distance by movement of the lens closer to the retina. This is brought about by 

 the contraction of a muscle called the retractor lentis (see Fig. 252A). In snakes the eye 

 at rest is also far-sighted. It is accommodated for near vision by the contraction of a 

 circular ring of muscle which compresses the eye and makes the lens, travel forward 

 (Fig. 252c). In birds the eye at rest is long-sighted. The focussing of near objects is 

 obtained by increasing the curvature of the cornea. This is caused in the following 

 manner. Attached to .the inside of the sclerotic, which forms a complete bony ring 

 round the eye, is a radially arranged muscle (Crampton's muscle). (See Fig. 252fi.) 

 The other end of this muscle is attached to the corneo-scleral junction. Therefore when 

 the muscle contracts it draws the periphery of the cornea backwards. But this tends 

 to cause an increase in the intraocular tension, since the total volume of the eye 

 tends to be decreased. This increased tension causes a bulging of those external eye 

 structures which are most elastic, namely to a slight extent the sclerotic where it 



Fia. 252. The methods of accommodation used in fish, birds and snakes. 



does not contain bone, but to a much greater extent the front of the* cornea, because 

 of its thinness and greater elasticity. The curvature and therefore the refract ing power 

 of the comea very greatly increase, thus causing light rays from near objects I.. IK- 

 focussed sharply on the retina. The increase in the distance of the cornea from the 

 retina still further assists this process. It should be noted that Crampton's muscle 

 contains voluntary fibres and is under the direct control of the will. This probably 

 serves two purposes : it allows the bird to rapidly accommodate as it swoops towards 

 the ground, and at the same time it may assist the judgment of distance. 



In describing the comparative anatomy of the visual organs it should be remem- 

 bered that the pineal gland is, in mammalia, the rudiment of one of a pair of median eyes 

 or ocelli, which were functional in the vertebrate ancestors. 



MINUTE ANATOMY OF THE EYE. THE CORNEA forms the trans- 

 parent anterior convex front of the eye. Its curvature has a radius of 

 nearly 8 nim. and a diameter of 11 mm. Its thickness is I'l mm., 

 and is composed of the following five layers: (1) Stratified epithelium 

 continuous with that covering the conjunctiva. Superficially the cells 

 are nucleated squaw, deeply they ;m> nucleated rolmminr cells, and 



