PRELIMINARY ANATOMICAL AND HISTOLOGICAL OBSERVATIONS. 817 



penetrate into the epithelial layer, again divide, ascending perpendicularly, and 

 end finally between the epithelial cells as minute fibers with small knobs (visible 

 on treatment with gold chlorid) (Fig. 334). The trophic fibers of the cornea are 

 probably the deeper-lying twigs that are connected with the corneal corpuscles. 



Blood-vessels are present only in the outer edge of the cornea (Fig 267, v], 

 and extend inward from the limbus a distance of 2 mm. above, 1.5 mm. below, 

 and i mm. laterally; the outermost capillary loops bend backward in an arched 

 manner. The cornea is nourished from its outer margin. Opacities of the cornea 

 produce corresponding disturbances of vision; pathologically, blood-vessels may 

 be formed within it. 



The cornea contains collagen and mucin (but no chondrin) ; the anterior 

 epithelium two globulins. The "membranin" of Descemet's membrane stands 

 between elastin and mucin, and is digested by trypsin. 



The sclera is a dense, fibrous tunic composed of connective-tissue bundles, 

 running in an equatorial (p) and a meridional (o) direction, with which are asso- 

 ciated many elastic fibers. In its interstices, which communicate with those 

 of the cornea, there are flat connective-tissue corpuscles, some of which are color- 

 less, some pigmented, and also wandering lymph-cells. It is thickest posteriorly, 

 and thinnest in the equatorial region; further forward it becomes thicker at the 

 point of insertion of the tendons of the four recti muscles. It contains only a few 

 blood-vessels, which form a wide-meshed capillary network immediately under 

 its inner surface. Other vessels form an arterial circle around the optic -nerve 

 entrance. In rare instances it is spherical, but usually it is more like an ellipsoid, 

 which must be conceived as produced by the rotation of an ellipse either about 

 its short axis (short eyes), or about its long axis (long eyes). Above and below, 

 the sclera overlaps the transparent corneal margin, so that the cornea has an 

 elliptical form if viewed from in front, and a circular form when viewed from 

 behind. Following the margin of the cornea, but within the scleral substance, 

 runs a circular canal, the canal of Schlemm (i), which anastomoses with other 

 venous channels (Leber's venous plexus) (&) ; Schwalbe and Waldeyer regard 

 Schlemm's canal as a lymph-channel. Posteriorly the sclera is continuous with 

 the sheath of the optic nerve derived from the dura mater. The sclera also 

 possesses nerves, which are said to terminate in the cellular elements within its 

 substance. 



II. Median or vascular tunic of the bulb, consisting of the choroid, the ciliary pro- 

 cesses, and the iris. 



The choroid is composed of the following layers: (i) On its inner surface there 

 is a transparent boundary layer, only 0.7 // thick, which becomes somewhat 

 thicker anteriorly. (2) The extremely vascular capillary network of the chorio- 

 capillary layer or membrane of Ruysch, embedded in a homogeneous layer. 

 Bounding this is: (3) A dense network of elastic fibers, which is lined on both 

 surfaces by endothelium. Then follows (4) the choroid proper, a layer with 

 pigmented connective-tissue corpuscles, which in the form of an elastic network 

 contains numerous veins, with their accompanying lymph-sheaths, as well as 

 arteries, which are provided with unstriated muscle-fibers in their connective- 

 tissue sheaths. Finally, there is (5) the supra-choroid layer or lamina fusca, 

 which bounds the large perichoroidal lymph-space (q) ; the latter is lined with 

 endothelium and is crossed by branched and anastomosing trabeculae covered with 

 endothelial and connective-tissue cells. In newborn infants, who always have 

 dark-blue irides, the uveal tissue contains no pigment; in bruns the pigment 

 develops later, while in blonds the uvea remains unpigmented. 



In the ciliary portion of the uveal tract, the pigmented connective-tissue cells 

 are not so numerous. In this position lies the ciliary muscle (muscle of accom- 

 modation or tensor of the choroid), whose meridional fibers (s) arise by means 

 of a branched, reticulated, connective-tissue insertion at the inner side of the 

 corneo-scleral margin, near Schlemm's canal, and extend backward into the 

 choroid; the radial fibers pass inward toward the interior of the eyeball; and the 

 circular bundles (0 are situated more internally, just within the ciliary border 

 (Heinr. Miiller's muscle). The motor nerve of this unstriated muscle is the oculo- 

 motor. Within the ciliary processes ganglion-cells have been found, which prob- 

 ably belong to the trigeminus. 



The iris consists of the following layers, from before backward: The anterior 



epithelium, a single layer of cells (v) ; a stroma with connective-tissue fibers and 



cells (vascular layer) ; and. finally, a posterior, structureless limiting membrane 



(membrane of Bruch), which is covered with the double layer of pigment-cells 



52 



