THE FROG 



97 



o.s. 



R.A. 



for distant view and more convex for things close at hand. This 



alteration, termed accommodation, is brought about by means 



of the ciliary muscles. 



Under normal conditions A 



the suspensory ligament is 



pulled tight, and so the 



lens is compressed by its 



capsule. When a near 



object is examined the 

 ciliary muscles contract, so R.F? 

 pulling the choroid for- 

 ward, thereby relaxing the 

 ligament and allowing the 

 lens by its own elasticity 

 to assume a more curved 

 shape. When the image, 

 which is inverted, falls on 

 the retina it is, as it were, 

 analysed by the rods and 

 cones and then transmitted 

 to the brain, by the relays 

 already noted, and there it 

 is interpreted. With the 

 advance of age in ourselves, 

 there is usually a gradual 

 increase in the rigidity of 

 the lens, and consequently 

 it becomes harder to see 

 things close at hand, so 

 that glasses have to be 

 worn when reading, etc. 



In order to point the 

 eye in the right direction, 

 it is provided with a series 

 of muscles which are ar- 

 ranged in two groups. The 

 first is a group of four mus- 

 cles, arising close together 

 at the inner posterior angle 

 of the orbit and inserted on 

 the top, bottom, inner and 



outer sides of the eyeball. They are termed collectively the recti 

 muscles, and individually the Rectus superior, Rectus inferior, Rectus 

 internus or anterior and Rectus externus or posterior respectively. 



R.s. 



3. 



R.R 



R.I. 



FIG. 33. Diagram of the muscles of the 

 eye of a vertebrate, Scyllium. 



A, eyeball removed, viewed from side ; B, viewed 

 from above; C., cranial wall; E., eyeball; I., iris; 

 O.T., inferior oblique ; O.S., superior oblique ; P., pupil ; 

 R.A., anterior or internal rectus ; R.I., inferior rectus ; 

 R.P., posterior or external rectus ; R.S., superior 

 rectus ; 3, 4, and 6, third, fourth, and sixth cranial 

 nerves. 



