THE NERVES OF THE CORNEA. 



735 



the epithelium, perforate Descemet's membrane, and thus communicate with the corneal 

 spaces. 



[Bowman's tubes are artificial productions, formed by forcing air or a coloured fluid between 

 the lamellae, when it passes between the bundles of fibrils, forming a series of tubes with dilata- 

 tions on them and running at right angles to one another between the lamellae. ] 



The nerves of the cornea, which are derived from the long and short ciliary nerves ( 347), 

 are partly sensory in function. They enter the cornea at its margin as medullated fibres, but 

 the myelin soon disappears, while the axial cylinders split up into fibrils. [The axial cylinders 

 branch and form a plexus between the lamellae, especially near the anterior surface, the funda- 

 mental or ground plexus (fig. 515, n). There are triangular nuclei at the nodal points, but 

 they probably belong to the sheath of flattened cells which cover the larger branches. There 

 is a finer and denser plexus of fibrils immediately under the anterior epithelium, sub-epithelial 

 plexus, which is derived from the former, the fibrils arising in pencils or groups (fig. 516). Some 

 fibrils perforate the anterior elastic lamina, rami perforantes, and pass between the anterior 

 epithelial cells to form the intra-epithelial network (fig. 515, b, p). Some observers suppose 

 that they terminate in free, 

 pointed, or bulbous ends. 

 There is also a fine plexus of 

 fibrils in the posterior layers 

 of the cornea, near Descemet's 

 membrane. It gives off nu- 

 merous fine fibrils, which come 

 into intimate, if not direct, 

 anatomical relation with the 

 corneal corpuscles. The trophic 

 fibres of the cornea ( 347) are, 

 perhaps, those deeper branches 

 which are connected with the 

 corneal corpuscles.] 



[Method. These fibrils are 

 best revealed by staining a 

 cornea with chloride of gold, 

 which tinges them of a purplish 

 line after exposure to light 

 (Cohnheim).] 



Blood-vessels occur only in 

 the outer margin of the cornea 

 (fig. 518, v), and extend 2 mm. 

 over the cornea above, 1 "5 mm. below, and 1 mm. laterally the most external capillaries form 

 arched loops, and thus turn on themselves. The cornea is nourished from the blood-vessels in 

 its margin. Opacities of the cornea give rise to many forms of visual defects. 



The sclerotic is a thick fibrous membrane, composed of, p, circular (equatorial) and, o, longi- 

 tudinal (meridional) bundles of connective-tissue woven together (fig. 514). The spaces between 

 the bundles contain colourless and pigmented connective-tissue corpuscles and also leucocytes. 

 It is thickest posteriorly, thinner at the equator, while in front of this it again becomes thicker, 

 owing to the insertion of the tendons of the straight muscles of the eyeball. It contains few 

 blood-vessels, which form a wide-meshed capillary plexus, immediately under its deep surface. 

 Other vessels form an arterial ring around the entrance of the optic nerve. It rarely is quite 

 spherical ; it rather resembles an ellipsoid, which we might imagine to be formed by the rota- 

 tion of an ellipse around its short axis (short eyes) or around its long axis (long eyes). Above 

 and below, the sclerotic overlaps like a fold the clear margin of the cornea ; hence, when the 

 cornea is viewed from before, it appears transversely elliptical, when seen from behind, it 

 appears circular. Following the margin of the cornea, but lying still within the substance of 

 the sclerotic, is the circular canal of Schlemm (i), which communicates with other anastomosing 

 veins, the venous plexus of Leber (k). Schwalbe and Waldeyer regard Schlemm's canal as a 

 lymphatic. Posteriorly, the sclerotic becomes continuous with the fibrous covering of the optic 

 nerve derived from the dura mater. The sclerotic is provided with nerves, which are said to 

 terminate in the cells of the scleral substance (Helfreich). 



The tunica uvea, or the uveal tract, is composed of the choroid, the ciliary part of the choroid, 

 and the iris. 



The choroid is composed of the following layers (fig. 517) : (1) Most internally is the 

 transparent limiting membrane, 07 fi in thickness, but it is slightly thicker anteriorly. (2) 

 The very vascular capillary network of the chorio-capiUaris, or membrane of Ruysch, embedded 

 in a homogeneous layer. Then follows (3) a layer of a thick elastic network, covered on 

 both surfaces by endothelium (Sattler). (4) The choroid proper consists of a layer with 

 pigmented connective-tissue corpuscles, together with a thick elastic network, containing 

 the numerous venous vessels as well as the arteries. The pigmented layers are known as the 



Vertical section of the cornea stained with gold chloride, n, 

 nerve-fibrils ; a, perforating branch ; r, nucleus ; p, b, inter- 

 epithelial termination of fibrils ; s, anterior elastic lamina. 



