Remarks on the Retina. 279 
ramifications. Immediately under the fibres there is a layer of pulp 
thinner than that of fishes. 
(When the sclerotic and choroide coats are removed, a little alco- 
hol poured on the exposed retina, and the preparation immersed for 
some hours in water, the membrane of Jacob is seen floating in the 
manner in which it is usually described. Beneath this, a cobweb-like 
membrane of considerable tenacity may be brushed off with a hair pen- 
cil. When this preparation is allowed to putrefy and the retina care- 
fully washed away with a soft brush, the vascular coat remains over 
the vitreous humor and may be easily separated fromit. Its nume- 
rous vessels ramify and anastamose with each other in the delicate 
membrane like the veins in a leaf.) 
In birds, the optic nerve before dividing, forms a line on each side 
of the marsupium, from which the fibres originate. 
In man, the fibres of the retina on the side towards the nose, radi- 
ate asin the ox. On the outer side some proceed no farther than 
the central foramen ; others pass beyond it, and after making a curve 
return and converge at the center, so that this point is surrounded by 
the extremities of fibres.* The layer of pulp around it is thicker 
than in any other part of the retina. 
This disposition of the fibres explains why there is a fold in the 
retina after death. It will naturally project at the weakest point 
when collapsed by the transudation of the humors. 
The rays of light fallimg upon the fibres and impelling them 
against their pulpy bed seem to be the cause of vision. ‘This may 
be proved by an experiment related by Sir Charles Bell. <‘ Close 
the eyelids and cover them witha piece of black cloth or paper that 
has a small hole in it, and place this hole, not opposite to the pupil 
but to the white of the eye; direct a beam of light upon the hole, 
and a person will see this light in its true direction.” In this exper- 
iment the light falls upon two parts of the retina. ‘The same or a 
greater impulse is given to the fibres first struck, but there is not a 
double impression because the fibres impinge against the vascular 
membrane and not against the pulp. 
The sense of touch is keenest at the termination of the nervous 
fibres at the pomts of the fingers, and that of taste at their termina- 
tion at the papille of the tongue. The sense of sight is also more 
acute at the termination of the fibres at the central foramen. 
The central foramen appears to be formed for the purpose of ena- 
bling those animals that possess it to view very minute objects. 
* The fibres are best seen in young animals. 
