930 THE SENSORY APPARATUSES. 



The choroid is a thin, dark-coloured membrane spread over the itmer face of 

 the sclerotic, the general conformation of which it repeats. It is divided into 

 two zones by the ora serrafa — a denticulated line which corresponds to the point 

 where the retina changes its characters. 



Posterior or choroid zone — Throughout the whole of this zone the choroid is 

 uniformly thin, and corresponds, by its external face {superficial layer, or stratum 

 of the larger vessels), with the sclerotic ; by its internal face, it is in contact with 

 the retina, but does not adhere to it. Posteriorly, it shows an opening through 

 which the optic nerve passes. In front, at the anterior opening of the sclerotic, 

 it is continuous with the anterior zone. 



The inner face (or layer — the memhrana-chorio-capillaris) of the choroid is 

 not uniform in colour, being perfectly black in the lower part of the eye ; this 

 is abruptly terminated at a horizontal line that passes about the eighth or ninth 

 part of an inch above the optic papilla. From this line, on the segment of a 

 circle from f to f of an inch in height, it shows most brilliant colours — at first 

 blue, then an azure-blue, afterwards a brownish blue, and beyond this an intense 

 black. The bright portion is the tapetum {lucidum, or tapetum fibrosum). 



The background of the colour is green with bluish shades, that vary slightly 

 according to the tint of the animal's coat. In black or dark grey Horses, the 

 tapetum has a deeper blue tint than in others ; and in white Horses it is some- 

 times yellowish brown. 



(This beautiful iridescent layer is composed of numerous undulating bundles 

 of connective tissue, contained in another layer of fine elastic fibres — the boundary 

 stratum of the ground substance — placed between the two layers of the choroid.) 



Anterior or ciliary zone. — This includes two parts— the " ciliary muscle " 

 and the " ciliary body." The ciliary muscle (anmdus albidus, or musculus 

 ciliaris) varies in width from one to two millimetres ; its external face adheres 

 closely to the sclerotic, and its internal is continuous with the ciliary body ; the 

 posterior border is continuous with the choroid zone, near the canal of Fontana. 

 The anterior border gives attachment to the greater circumference of the iris. 

 Its structure and uses will be referred to hereafter. 



The ciliary body (corpus ciliare) forms a kind of zone or ring, wider than the 

 ciliary muscle, and consequently overlaps the latter before and behind. It 

 extends, on one side, on the inner face of the choroid, and on the other, on the 

 posterior face of the iris. When the cornea and sclerotic are removed so as 'to 

 expose the ciliaiy muscle, this zone is not seen ; and to discover it, it is necessary 

 to excise all the posterior part of the shell of the eye by a circular incision, and 

 evacuate the vitreous humour. "We then observe, around the crystalline lens, 

 a wide, black circle, forming very regular radiating folds, projecting inwards by 

 their inner extremities, and appearing in the posterior chamber of the eye, after 

 cutting away the iris ; all abut by these extremities on the circumference of the 

 lens, which they do not quite reach, although the latter is sustained by, and 

 " set" in, the centre of the ciliary body. 



These radiating folds, named the ciliary processes (Fig. 503, 4), are from 

 1 1 to 1 20 in number in the Horse, and are constituted by little parallel leaves, 

 wider at their inner than their outer extremity ; the furrows that separate them 

 posteriorly are partly concealed by the prolongation of the retina that constitutes 

 the zonula of Zinn. The coronet formed by the ciliary processes is usually 

 asymmetrical — that is, narrower on the inner than on the outer side. 



(The elements of the ciliary muscle run in three directions : 1. Meridional 



