1094 



ORGANS OF SPECIAL SENSE 



FIG. 799. From a horizontal section of an ox's cornea. Positive 

 picture of the canal systeri demonstrated by the gold chloride 

 method. X 450. (Szymonowicz.) 



innermost, when separated from the proper substance of the cornea. Its use appears to be 

 "to preserve the requisite permanent correct curvature of the flaccid cornea proper" (Jacob). 



At the margin ofthe cornea this posterior elasticinembrane breaks up into fibres to form a 

 reticular Wucfure^f^he outer angle (fWHWfrfllfloWfflfflrrJlrTfiie intervals between the fibres 



forming small cavernous spaces, 

 Corneal cell in -f the spaces of Fontana (spatia 



finaliculi lymph space ancfuli iridis) (Fig. 7&4). These 



tgM ' i lil lie spaces communicate with a 



circular canal in the deeper parts 

 of the corneoscleral junction. 

 This is the canal of Schlemm 

 (sinus venosus sclerae) (Figs. 700 ?< 

 and 764) ; it communicates inter- 

 nally with the anterior chamber 

 through the spaces of Fontana, 

 and externally with the scleral 

 veins. Some of the fibres of this 

 .reticulated structure are continued 

 ' into the front of (he iris, forming 

 the ligamentum pectinatum iri- 

 dis; while others are connected 

 with the fore part of the sclera 

 and choroid. 



5. The posterior layer or the 

 corneal endothelium (endothelium 

 camerae anterioris) lines the 

 aqueous chamber and prevents 

 the absorption of the aqueous 

 humor. It covers the posterior surface of the elastic lamina, is reflected upon the front of 

 the iris, and also lines the spaces of Fontana. It consists of a single layer of polygonal, flattened, 

 transparent, nucleated cells, similar to those lining other serous cavities. 



Arteries and Nerves. The fetal cornea contains bloodvessels which pass from'the margin 

 almost to the centre. The adult cornea contains no bloodvessels, except at its margin^ The 



capillaries from the sclera and conjunctiva form loops at the corneal margin, and many of these 

 loops enter the cornea for a distance of 1 mm. (Fig. 802). The balance of the cornea is non- 

 vascular and obtains its nourishment from the lymph in the lacunae and canaliculi. Lymphatic 

 vessels have not as yet been demonstrated in it, but are represented by the channels in which 

 the bundles of nerves run; these channels are lined by endothelium and are continuous with the 

 cell spaces. The nerves are numerous, and they are derived from the ciliary nerves; they form 

 the annular plexus (plexus annularis), at the corneal margin, and enter the laminated tissue of 

 the cornea, lose their myelin sheaths, and ramify throughout the substantia propria as the plexus 

 of the stroma. From this deep plexus come perforating fibres (filrrae perforantes), which pass 

 through the anterior elastic lamina and form the subepithelial plexus, and from it fibres are 

 given off which ramify between the epithelial cells, forming a network which is termed the intra- 

 epithelial plexus. Nerve fibres from the annular plexus and from the plexus of the stroma come 

 into close relation with the corneal corpuscles. 



Dissection. In order to separate the sclera and cornea, so as to expose the second tunic, 

 the eyeball should be immersed in a small vessel of water and held between the finger and thumb. 

 The sclera is then carefully incised, in the equator of the globe, till the choroid is exposed. One 

 blade of a pair of probe-pointed scissors is now introduced through the opening thus made, 

 and the sclera divided around its entire circumference, and removed in separate portions. The 

 front segment being then drawn forward, the handle of the scalpel should be pressed gently 

 against it at its connection with the iris, and, ,these being separated, a quantity of perfectly trans- 

 parent fluid will escape; this is the aqueous humor. In the course of the dissection the ciliary 

 nerves (Fig. 800) may be seen lying in the loose cellular tissue between the ehoroid and sclera 

 or continued in delicate grooves on the inner surface of the latter membrane. 



II. The Choroid, Ciliary Body, and Iris (Tunica Vasculosa Oculi) 



(Figs. 800, 820). 



The middle tunic of the eye is formed from behind forward by the choroid, 

 the ciliary body, and the iris. 



The choroid invests the posterior five-sixths of the globe, and extends as far 

 forward as the ora serrata of the retina; the ciliary body connects the choroid 



