742 THE SENSE ORGANS AND SKIN OF THE HORSE 



tion of the next layer. (3) The substantia propria forms the bulk of the cornea 

 and is composed of interlacing bundles of connective tissue, arranged in part in 

 lamellae disposed parallel with the surface. In the amorphous cement substance 

 between the lamellae are flattened connective-tissue cells, the corneal corpuscles. 

 These have branching processes which unite with those of other cells, thus form- 

 ing a protoplasmic network.^ ("4) The lamina elastica posterior, also termed the 

 membrane of Descemet or Demours, is a thin and practically homogeneous mem- 

 brane which is less intimately attached to the substantia propria than the anterior 

 lamina. It is clear, glistening, and elastic. At the periphery the lamina divides 

 into three sets of fibers. The anterior fibers join the sclera, the middle give attach- 

 ment to the ciliary muscle, while the posterior pass into the iris and form the 

 ligamentum pectinatum iridis. (5) The endothelium (camerae anterioris) consists 

 of a layer of flattened polygonal cells, and is reflected on to the anterior surface of 

 the iris. 



The cornea is without blood-vessels except at its periphery', where the terminal 

 twigs of the vessels of the sclera and conjunctiva terminate in loops. The nerves 

 are derived from the ciliary nerves. They form a plexus around the periphery 

 (Plexus annularis), from which fi])ers pass into the substantia propria, become non- 

 medullated, and form the fundamental or stroma plexus. From this perforating 



branches pass through the anterior limiting layer and 

 form a subepithelial plexus, from which filaments ascend 

 / ----5>. between the epithelial cells. Other branches from the 



plexuses in the su1:>stantia propria end as fibrils which are 

 in close relation with the corneal corpuscles. 



Fig. 55S. — Tapetum of Horse. 



The Vascular Tunic 

 The vascular tunic (Tunica vasculosa oculi) lies in- 

 ternal to the fibrous coat; it comprises three parts- — the 

 chorioid, the ciliary body, and the iris. 



1. The chorioid (C'horioidea) is a thin membrane 

 ^ ,. .,, , , which lies between the sclera and retina. It is in 



a. Optic papula; b, lower 



border of tapetum. (After Ellen- general rather loosely attached to the sclera by the 

 berger, in Leisering's Atlas.) lamina fusca, but is intimately adherent at the point 



of entrance of the oj^tic nerve and less closel}^ in 

 places where the ciliary vessels and nerves i)ass through. The inner surface 

 is in contact with the layer of pigmented cells of the retina, which adhere so 

 closely to the chorioid that they were formerly regarded as a part of the latter. 

 The general color of the chorioid is dark brown, but an extensive semilunar area a 

 little above the level of the optic papilla has a remarkable metallic luster, and is 

 termed the tapetum of the chorioid (Tapetum chorioideae). The appearance here 

 varies in different individuals, but the prevailing colors in most cases are iridescent 

 blue and green in various nuances shading into yellow. Posteriorly the chorioid is 

 perforated by the optic nerve, and anteriorly it is continuous with the ciliary bod3\ 

 The chorioid consists of four layers, which from without inward are as follows: 

 (1) The lamina suprachorioidea consists of interlacing fine lamellae of fibrous tissue, 

 each containing a network of elastic tissue. Among these are large-branched, pig- 

 mented, connective-tissue cells. The spaces between the lamelhe are lined with 

 endothelium, and form a system of lymph-clefts which togctlKn- form the pericho- 

 rioidspacc^ (Spatium i^erichorioidcale). (2) The lamina vasculosa is tlie outer part 



1 According to Piersol tho system of spaces and canaliculi in the substantia propria is com- 

 pletely filled by the cells and their processes, upon which the nutrition of the cornea largely de- 

 pends. The lamina elastica anterior, formerly described as a distinct layer between the corneal 

 epithelium and the substantia propria, does not exist as such, but there is a condensation of the 

 superficial part of the latter, which Rollett termed the anterior limiting layer; it is not elastic. 



