NATURAL PHILOSOPHY. 151 



printed), containing the result of the measurement of two hundred eyes of vari- 

 ous creatures. In conclusion, Mr. Xunneley said : The measurements, I think, 

 clearly prove that whatever part the fibres of the optic nerve play in the 

 phenomena of vision, and they, in all probability, only convey to the sen- 

 sorium the impression received by the true retinal elements, the greatest 

 number of them are distributed on that part of the eyeball where there is the 

 greatest range of vision, and that the largest expanse of retina is on that 

 part of the ball opposite to where objects are placed, and consequently it is 

 where the visual images of them must fall. Thus the extent of vision is 

 always in conformity with the space of retina on that side of the optic nerve, 

 and as the rods and cellules appear always to correspond in abundance with 

 the fibres, that side of the retina which receives the greatest number of im- 

 ages is most exercised, or where the range of vision is the greatest, is always 

 the largest. That this is a fact, I think a careful comparison of the position 

 of the eyes in the head, the size of the eyeball, and the exact position of the 

 entrance of the nerve into it, with the mode of life and habits of various 

 creatures, will render more obvious than a casual glance would do. To men- 

 tion only a few instances as illustrations: Man, from the erect position of 

 his body, the horizontal placing of his eyes, and his habits, has a more pan- 

 optic range than any other creature (of course in this consideration all mo- 

 tion of the head, neck, and body of the animal, must be excluded, and 

 those of the eyeballs alone admitted). In him the optic nerve enters the ball 

 not far from the centre ; leaving, however, a somewhat shorter space on the 

 inner and lower parts of the retina than on the upper and outer. Now, while 

 man enjoys a free range of vision above the horizontal line, there arc far more 

 occasions for him to look at objects below than above this line, and thus mere 

 visual images are projected to the upper and outer sides of the entrance of 

 the optic nerve oftener than to the inner and lower sides of this spot. In the 

 pig, who sees at no great range before him, and who seeks his food with the 

 snout almost always in the ground, whose head and eyes are consequently 

 for the most part downward and near the ground, the nerve enters the ball 

 more outwardly and much lower than it does in man. The pig wants not to 

 see far before him, but he does require, while grubbing, to look behind him, 

 from whence danger comes. So with the timid herbivorous animals. Look 

 at the entrance of the nerve in the bullock and sheep, who pass so much 

 time with the head in a dependent position near to the ground with the eye 

 directed upon the surface, in open plains, where danger usually comes from 

 behind; in them the upper and the inner sides of the retina are much larger 

 than the lower and outer portions ; while in the deer, who live in more wooded 

 places, where danger is also from the front, but who, like the bullock, has 

 the head downward in feeding, though' the inner or anterior side of the retina 

 is still larger than the posterior, it is so to a much less extent than it is in the 

 bullock, while the upper portion still continues as proportionately large as it is 

 in sheep and bullocks. On the contrary, in the horse, who is not so preyed 

 upon, who carries the head erect, and observes all around, the nerve enters 

 the eye more nearly in the axis. In birds, with few exceptions, the upper 

 portion of the retina is much more considerable than the lower parts, but the 

 anterior and posterior portions A'ary much in different genera. Those whose 

 locomotion is performed principally by the feet, and whose range of habita- 

 tion is very small, as the common fowl and turkey, have the inner or anterior 

 portion very considerably greater than the outer or posterior. Those birds 

 whose range is greater, and who use the wings for progression, but do not 



