148 OUTLINES OF EQUINE ANATOMY. 



terior surface of tlie iris, and gives tlie eye its colour 

 (deficiency of this constitutes " wall eyes "). The 



Iris is a muscular diaphragm, which projects into the 

 cavity of the eyeball, leaving centrally an oblong opening, 

 the pupillary opening or pupil. It consists of two orders 

 of unstriated muscular fibres : — circular, surrounding the 

 pupil, and which cause it to contract; radiating, which 

 extend from the outer circumferent margin to the pupil, 

 which they serve to enlarge. These are posteriorly covered 

 by the uvea, which overhangs the pupil above and below, 

 forming dark pigmentary masses visible in the living sub- 

 ject, corpora nigra. They are largest at the upper margin 

 of the pupil, and frequently are wanting below. The 

 anterior and posterior surfaces of the iris are covered by 

 serous membrane continuous around the margin of the 

 pupil. The outer margin of the iris is attached to the 

 ciliary ligament, a structure containing unstriated mus- 

 cular fibre, situated at the outer circumferent margin of 

 the posterior surface of the cornea, and serving to connect 

 together the cornea, sclerotic, iris, ciliary processes, and 

 choroid ; in its substance is the canal of Fontana. The 

 optic nerve runs from the optic decussation at the base of 

 the brain, through foramen optici, after emerging from 

 which it is pierced by arteria centralis retinae, a branch of 

 the ophthalmic. It runs through the orbit with this artery 

 in its centre, and invested by a stout sheath, which extends 

 from the dura mater to the sclerotic. After piercing the 

 lamina cribrosa, its fibres radiate on the inner surface of 

 the choroid, forming the middle layer of the retina, or 

 tunica nervosa, which extends as far as the outer margin 

 of the ciliary processes, ora serrata. On its external sur- 

 face it has the tunica Jacobi or bacillary layer, which con- 

 sists of nerve-cells (rods, cones, and ganglion cells) imbedded 

 in the pigmentary layer of the choroid. On its internal 

 surface are blood-vessels, which in the living subject may 

 be seen by means of the ophthalmoscope (tunica vasculosa) . 

 At the opening of the optic nerve into the eye, optic disc or 

 blind spot, visible on careful inspection of the living sub- 

 ject, nerve-cells are deficient, while there is a prepon- 

 derance of nerve-fibres. In the human subject the macula 

 lutea or foramen of Soemering presents exactly opposite 

 conditions, nerve-cells being here most numerous. We 

 have failed to distinguish it in the horse. The different 



