BY THE PRESIDENT. 
115 
is distinct from that of the eye usually found in Invertebrata.* 
In the latter case the nerve becomes differentiated into its terminals 
as it approaches the optic chamber, the hinder portion of which is 
lined with nothing but the nerve-terminals which point towards 
the light; on the other hand, in the Yertebrata the optic nerve 
becomes differentiated with terminals only after it has entered the 
optic chamber or eye-ball and after it has commenced its retinal 
expansion, and the nerve-terminals, namely, the rods and cones, 
point inwards, i.e. away from the light. Hence there is a spot 
on the retina of the vertebrate eye where there are no nerve- 
endings, or in other words where there is not the structure 
essential for the production of a visual sensation. Thus even in 
the human eye there is a blind spot to which I shall afterwards 
refer. The most acute vision of the eye is situated where a 
line would meet the retina if drawn through the centre of the 
cornea and the centre of the lens. This line is called the optic 
axis, and at the point where it meets the retina there is, in the 
human eye and in no other, a slight circular depression of a 
yellowish hue called the yellow spot. It is there that the nerve- 
endings are the most numerous, but instead of the rods and cones 
being uniformly distributed as in other parts of the retina, the 
cones are abundant, while the rods are found only towards the margin 
of the depression. When we look at anything, we place our eye 
in such a position that the image falls on the yellow spot; this is 
called “direct vision.” The term “indirect vision” is applied 
when the image falls on the other parts of the retina, in which 
case it is much less distinct. 
The various habits of the Yertebrata necessitate some adjustment 
in their visual organs. Thus the fish, living in a dense medium, 
has a different lens from that of a bird with a wide field of vision. 
The Eyes oe Eishes.| —The eyes of Myxine , Petromyzon , and 
Amblyopsis are hidden. In Amphioxus they are represented by 
a mere pigment-spot lying directly on the nervous system. The 
function of such eyes must be most limited. In all other fishes 
the eye is well developed, but the muscles which serve for adjust¬ 
ment are feeble. The cornea is comparatively flat, and part of the 
lens projects through the pupil, which is exceptionally large, while 
the iris is capable of but very slight contraction and dilation. The 
outer covering of the eyeball, or the sclerotic, contains some cartila- 
* The eyes of the pecten and the dorsal eyes of the Onchidia, to which I refer 
hereafter, resemble those of the Yertebrata in- this respect. The “ pineal eye ” 
of reptiles mentioned in the conclusion of my address is of the Invertebrate type. 
f See “ Beitrage zur Anatomie des Sehorganes der Fisches,” by Berger, 
‘ Morphologisches Jahrbuch,’ Band, viii, in which there is a bibliography. 
