172 



ADAPTATIONS TO DIURNAL ACTIVITY 



merely by having a large eye whose parts are proportioned as they are in 

 small twenty-four-hour eyes. It is only in large animals such as the ungu- 

 lates and the great cats that we find high visual acuity attributable prin- 

 cipally to large ocular size as such. 



Where the habits of the animal demand that he go all out for visual 

 acuity, we find the eye to be large both absolutely and relatively. Thus in 

 the birds the eyes are proportionately colossal and occupy so much of the 

 head (Fig. 70) that their fundi may actually roll upon one another in 



Fig. 70 — Eyes and brain of the English sparrow, Passer domesttcus, in situ, from the ventral 

 side. X 5^^. Redrawn from Wood and Slonaker, 



c- optic chiasma; e- external rectus; g- Gasserian ganglion; h- Harderian gland; in- 

 inferior rectus; io- inferior oblique; ir- internal rectus; /- lacrimal gland; m- medulla; 

 o- optic nerve; ol- optic lobe (midbrain); p- pituitary; 5- third cranial (oculomotor) 

 nerve — supplies the superior, internal, and inferior reai and the inferior oblique; 4- 

 fourth cranial (trochlear or pathetic) nerve — supplies the superior oblique; 5- fifth cranial 

 (trigeminal) nerve, several of whose branches carry fibers to the eye and adnexa; 6- sixth 

 cranial (abducens) nerve — supplies external reaus. 



the mid-plane of the skull, like a pair of segmental gears. Only in species 

 of little brain can such things be. 



The partial dependence of resolving power upon absolute ocular size 

 has a consequence upon relative ocular size. It has been stated as a law 

 that eye size is inversely proportional to body size (Haller's ratio) . The 

 reason why this should hold for nocturnal forms as well as for diurnal 

 ones will be given in the next chapter; but it is a very different reason. 

 Keeping only diurnal forms in mind, it is easy to see why the eye should 



