216 COMPARATIVE ANATOMY 



are situated towards the choroid, or are external. Thus the 

 terminal elements of the neuro-epithelium are turned away 

 from the rays of light falling upon the retina, and the rays must 

 therefore pass through all the other layers before they reach the 

 rods and cones. 



Fishes possess the longest, Amphibians the thickest rods, so that in the 

 latter there are only about 30,000 to a square millimetre, while in Man there 

 are from 250,000 to 1,000,000. 



In Fishes the rods far exceed the cones in number, while in Reptiles and 

 Birds the reverse is the case. The cones of many Reptiles and all Birds are 

 distinguished by the presence of brightly coloured oil-globules, which are 

 also present in those of Marsupials. 



In the centre of the retina of higher Vertebrates there is a specially 

 modified region of most acute vision, called the yellow-spot (fovea 

 centralis or macula lutea). It is due to the thinning-out of all the 

 layers except that of the rods and cones, and even the rods disappear, only 

 the cones persisting (Fig. 168). 



Accessory Organs in Connection with the Eye. 



(a) EYE-MUSCLES. 



The movement of the eyeball is always (except in Myxinoids, 

 comp. p. 211) effected by six muscles, four of which are known 

 as the recti (superior, inferior, anterior or internal, and posterior or 

 external), and two as the dbliqui (superior and inferior). The 

 former, which arise from the inner portion of the orbit, usually 

 from the dural sheath of the optic nerve, together circumscribe a 

 pyramidal cavity, the apex of which lies against the inner portion 

 of the orbit, while the base surrounds the equator of the eyeball, 

 where the muscles are inserted into the sclerotic. 



Both the oblique muscles usually arise from the anterior or 

 nasal side of the orbit, and as they respectively pass from this region 

 dorsally and ventrally in an equatorial direction round the eyeball, 

 they constitute a sort of incomplete muscular ring. 



A deviation from this arrangement is seen in Mammals, in which the 

 superior oblique has gradually come to arise from the inner part of the 

 orbit, and then passes forwards towards its anterior (internal) angle, where 

 it becomes tendinous, and passes through a fibre-cartilaginous pulley (trochlea) 

 attached to the upper border of the orbit, on the frontal bone. Hence it is 

 sometimes called the trochlear muscle. From this point it changes its direc- 

 tion, and becomes reflected obliquely outwards and backwards to the globe of 

 the eye. 



Besides these six muscles, others are usually present which 

 are known as the retractor lulli (best developed in Ungu- 

 lates), the quadratus (bursatis), and the pyramidalis. The last 

 two are connected with the nictitating membrane (see p. 217), 

 and are present in Reptiles and Birds. All three are supplied by 

 the abducent nerve (comp. p. 184). 



