256 DISEASES OF THE HORSE. 



are reflected by the surface of the earth ; by this admh-ablc pro- 

 vision he sees what it is necessary he should see, while the too 

 powerful stimulus of the direct rays from the sun are excluded, 

 or sufficiently moderated. These black bodies enable the animal 

 also to close the pupil completely, and thus occasionally, or when 

 the nervous structure of the eye is morbidly irritable, exclude 

 the light altogether. 



All that part of the eye which is posterior to the iris is chiefly 

 occupied by the thii'd or vitreous humour, and it is in this hu- 

 mour that the crystalline lens is imbedded. The vitreous 

 humour is perfectly transparent, and consists of a fluid, inclosed 

 in numerous small transparent cells, all of which appear to be 

 enveloped in one delicate transparent membrane, named tunica 

 arachnoidea. If the vitreous humour is cut, by snipping it with 

 scissors, a fluid, like water, drops from it freely, so that it ap- 

 pears to be nothing more than water, probably ht)lding a little 

 salt in solution, which escapes when the transparent cells are 

 thus cut open. 



That part of the vitreous humour in which the lens is im- 

 bedded is difi:erent from the other parts, and of the consistence 

 of jelly. Immediately behind the ciliary ligament, as it is 

 termed, the arachnoid coat may be inflated with a small blow- 

 pipe, and made to resemble a circular canal ; this has been 

 called, from the name of the person who first observed it, the 

 circular canal of Petit. It is supposed to be connected with the 

 radiated fibres of the iris. I have seen the whole of this humour, 

 in the eye of a sheep that had an hydatid in the right ventricle 

 of the brain, of the consistence of jelly. 



[The retina is a fine delicate transparent membrane, on which 

 a picture of external objects is painted. It is spread out on the 

 back part of the vitreous humour, and has the choroid coat im- 

 mediately behind, which therefore acts like the quicksilver of a 

 looking-glass in preventing the light from passing through. The 

 retina is supposed to be a continuation of the optic nerve. 



The choroid coat, whose use has been specified, appears to be 

 a net-work of blood-vessels, secreting a dark paint on both its 

 surfaces. It is analogous to the rete-mucosum of the skin, and 

 in the human subject is quite black, being that which gives the 

 colour to the pupil of the eye. In animals, however, we observe 

 a great variety in the colour of that portion of it immediately 

 opposite the pupil, or at the back of the eye, being in the cat of 

 a yellow colour, and in the horse of a greenish blue. The light 

 colour of this part serves to reflect some portion of the light 

 admitted to the eye, and thus economises and renders available 

 the partial light that is always present even in the darkest night ; 

 and thus it is that a horse is enabled to pick his way with ease 

 when his rider is unable to distinguish a single ofyect. The 



