810 THE OEGANS OF SENSE. 



2. The anterior elastic lamina is from 19-20 ^ thick, and is regarded merely as a differentia- 

 tion of the anterior part of the snbstantia propria, from which it is with difficulty separated ; it 

 is not stained yellow by picrocarmine, thus differing from true elastic tissue. Its degree of 

 development varies in different animals. 



3. The substantia propria presents, in a fresh condition, a homogeneous appearance ; but, 

 with the assistance of reagents, it is seen to consist of modified connective tissue, with a few 

 elastic fibres. An amorphous interstitial substance binds the fibres into bundles, and, in turn, 

 cements the bundles into lamellae which are flattened from before backwards. The fibres of any 

 one lamella cross those of adjacent lamellae almost at right angles, while the superimposed lamellae 

 are joined by sutural fibres and by amorphous substance. Between the lamellae are found the 

 cell spaces or lacunae of the cornea irregularly stellate in shape, and communicating freely 

 with each other by means of fine canaliculi. The corneal cells or corpuscles are contained in 

 these lacunae, without, however, completely filling them, the remainder of the cavities being 

 occupied by lymph. The cells are nucleated, flattened, and star-like, and their branched pro- 

 cesses join those of neighbouring cells in the canaliculi. Migratory or lymph cells are also found 

 in cell spaces. 



After middle age a grayish opaque ring, 1*5 to 2 mm. in breadth, is frequently seen near the 

 periphery of the cornea ; it is termed the arcus senilis, and results from a deposit of fat granules 

 in the lamellae and corneal corpuscles. 



4. The posterior elastic lamina is a clear homogeneous membrane, covering the posterior 

 surface of the substantia propria and possessing a thickness of 6-8 /* at the centre and 10-12 fj, at 

 the periphery of the cornea. Less firmly attached than the anterior elastic lamina, it may be 

 stripped off, when it will be found to roll up with its attached surface inwards. Between the 

 ages of twenty and thirty years small wart-like projections appear on its deep surface, near its 

 periphery, and these increase in size and number as years advance, so that in old age the 

 membrane may attain a thickness of 20 /*. At the sclero-corneal junction the posterior elastic 

 lamina splits into bundles of fine fibres which interlace and form the triangular area of trabecular 

 tissue already referred to (p. 808), and which is usually spoken of under the name of the 

 ligamentum pectination iridis. The meshes or spaces between the trabeculse are termed the 

 spatia anguli iridis (O.T. spaces of Fontana), and are lined with endothelium prolonged from the 

 endothelium of the anterior chamber. They communicate internally with the filtration angle 

 and externally with the sinus venosus sclerae, and form important channels through which fluid 

 may filter from the anterior chamber into the sinus and thence into the anterior ciliary veins. 

 When the trabecular tissue of the ligamentum pectinatum iridis is followed backwards most of 

 its fibres are seen to be attached to the anterior surface of an inwardly directed rim of scleral 

 tissue ; in a meridional section this rim appears as a triangular projection, and is named the scleral 

 spur. A few fibres of the trabecular tissue are carried past the apex of the scleral spur on to the 

 inner surface of the origin of the meridional fibres of the ciliary muscle, and, passing behind 

 the filtration angle, are prolonged into the iris (Fig. 678), where they are directly continuous with 

 the fibres of the dilatator pupillae muscle (Arthur Thomson). 1 



5. The endothelium of the anterior chamber consists of a single stratum of nucleated, 

 flattened, polygonal cells, which present a fibrillar structure and are continued as a lining to the 

 spatia anguli iridis ; this layer of endothelium is also reflected on to the anterior surface of the iris. 



Vascular and Nervous Supply of the Cornea. In the foetus the cornea is traversed, almost 

 as far as its centre, by capillaries ; but in the adult it is devoid of blood-vessels, except near its 

 margin. The capillaries of the conjunctiva and sclera pass into this marginal area for a distance 

 of about 1 mm., where they terminate in loops. All the remainder of the cornea is nourished by 

 the lymph which circulates in its cell spaces and canaliculi. 



The nerves of the cornea are derived from the ciliary nerves. Around its periphery they 

 form an annular plexus, from which fibres pass into the cornea, where, after a distance of 1 or 

 2 mm., they lose their medullary sheaths and ramify in the substantia propria, forming what is 

 termed the fundamental or stroma plexus. Fibres extend from this plexus through the anterior 

 elastic lamina and form a subepithelial plexus, from which fine filaments ramify between the 

 epithelial cells as far as the superficial layers. From the annular and stroma plexuses fibrils 

 pass to the substantia propria and come into close relation with the corneal corpuscles. 



TUNICA VASCULOSA OCULI. 



The middle, vascular, and pigmented tunic of the bulb of the eye comprises, 

 from behind forwards, the chorioid, the ciliary body, and the iris (Fig. 681). 



Chorioidea. The chorioid intervenes between the sclera and the retina, reach- 

 ing as far forwards as the ora serrata of the retina (p. 815). It is dark brown or 

 black in colour, and is thicker behind than in front ; posteriorly it is pierced by 

 the optic nerve, and is there firmly attached to the sclera. Its outer surface is 

 flocculent and is connected to the sclera by the loose lamina fusca; its inner 

 surface is smooth and is adherent to the outermost or pigmented layer of the retina. 



The chorioid consists of a loose connective tissue, embedded in which are blood- 

 vessels and branched pigment cells; from without inwards it consists of three 



1 Op. cit. 



