observations on soundness. 
259 
diffident in introducing the subject to the readers of the 
Veterinarian; there being innumerable and almost insur¬ 
mountable difficulties in my way. 
There are several modes employed by persons to examine 
the eye, but all of them aim at the same object, namely, that 
of ascertaining whether the eye be a sound one or not. I 
don’t believe I am far out when I assert that very few exami¬ 
ners trouble themselves about an eye, provided it is free 
from actual blemish; and if neither opacity of the transparent 
cornea nor of the lens is present, and the eye is pretty clear, 
as a general rule, the horse passes sound as regards the eyes. 
In order to make myself understood, it will be necessary to 
consider in the first place the structure of some of the parts ; 
afterwards to offer a few observations upon the eye as an optical 
instrument; otherwise, in all probability, some of the remarks 
may appear somewhat strange, and perhaps novel. 
The eye is divided into the globe and its appendages. The 
globe is composed of tunics and refracting media, called 
humours. The tunics are three in number, viz.— 1st, sclerotic 
and cornea; 2nd, choroid, iris and ciliary processes; 3rd, retina 
and zonula ciliaris. The humours are also three in nunrber— 
aqueous humour, crystalline lens, and the vitreous humour. 
The sclerotic coat and the cornea form the external tunic of the 
eyeball, and give to it its peculiar form; the sclerotic invests 
four fifths of the globe, and the cornea the remainder. The 
sclerotic is a dense, fibrous membrane, thicker posteriorly 
than anteriorly; it is continuous posteriorly with the sheath 
of the optic nerve, which is derived from the dura mater; it 
is pierced by the optic and ciliary nerves and ciliary arteries; 
anteriorly it presents a beveled edge for the reception of the 
cornea; the anterior surface of the sclerotic is covered by 
the tunica albuginea, which is formed by the expansion of 
the tendons of the four straight muscles; the tunica albuginea 
is partly covered by the conjunctiva; at the entrance of the 
optic nerve the sclerotic forms a thin, cribriform lamella, which 
is pierced by a number of minute openings for the passage 
of the nervous filaments; one of these openings is larger than 
the rest, and is situated in the centre of the lamella which is 
called the porus opticus ; it gives passage to the arteria centralis 
retinae. The cornea is the transparent projecting layer which 
forms the anterior fifth of the globe of the eye. In form it is 
circular, being concavo-convex ; it is received by its thin, sharp 
edge within the bevelled border of the sclerotic. It consists of 
four layers : first, the conjunctiva ; secondly, the cornea proper, 
which is constituted of several thin lamellae, connected together 
by an extremely fine, cellular tissue; thirdly, the cornea elastica, 
