222 
POPULAR SCIENCE REVIEW. 
of the optic nerve, wliicli perforates it, to the ciliary ligament 
(Plate XI., fig. 2, c, e, c), where it forms the iris and ciliary 
processes. The choroid consists almost entirely of blood- 
vessels, and colouring matter, or pigment, contained in irregu- 
larly-shaped cells in the interval between the blood-vessels. By 
washing gently with water, the pigment on the surface may 
be removed, and we then see the veins running in great 
numbers, and in a beautifully-arranged manner, from behind 
forwards. The irregularly parallel lines seen in Plate XI., 
fig. 2, are the veins. The innermost layer of the choroid consists 
of a series of little bodies, called epithelial cells, arranged in 
the manner of a pavement ; in the eye of man, and in the 
front of the eye of the ox, these are full of dark colouring 
matter (Plate XL, fig. 5, a). The back of the eye of the ox, 
however, instead of presenting a dull black appearance (as 
in man), is of a mingled blue and green colour, and very 
brilliant, having quite a metallic lustre. This is due to the 
presence of certain fibrous elements in the choroid, so arranged 
as to give the play of colours named : for there is, on examina- 
tion, no colouring matter to be seen ; the epithelial cells of 
this part having the microscopical appearance of b, Plate XI., 
fig. 5. This bright and coloured surface is of considerable 
extent, and is called the tapetum {tapete, a carpet). 
At c, Plate XI., fig. 2, is the cihary ligament and muscle. 
At this point the choroid bends inwards to form the cihary 
processes and iris. The cihary processes ( cilium , an eyelid 
or eyelash), (Plate XI., fig. 3, g, g, and the outer circle in 
Plate XI., fig. 9, where they are seen from behind), con- 
sist microscopically of the same structures as the choroid ; 
they are formed by the choroid bending inwards and back- 
wards from the ciliary ligament towards the lens (Plate XI., 
fig. 3, i ) ; together they constitute a circular band, with a 
large central aperture at its circumference continuous with 
the choroid, and also attached to the iris, but free at its 
margin and front surface. These processes consist of folds 
or plaits which are arranged around the lens, “ like the many 
petals of a flower •” the folds are both long and short : the 
long ones, about sixty in number, running concentrically along 
the entire breadth of the processes ; the short ones are placed 
alternately with the long ones, commencing- at the circum- 
ference, but terminating about halfway from the margin of 
the large central aperture. 
The consideration of the iris and ciliary muscle we will 
reserve for the present, and will examine the layer imme- 
diately beneath the choroid, namely, the retina ( rete , a net) 
(fig. 3, the scarlet line running around the globe). This 
is the nervous structure which receives the impression of light, 
