PRACTICAL CONSIDERATIONS: THE FIBROUS TUNIC. 1453 



and supply the posterior layers. P'ully two-thirds, however, after forming a funda- 

 mental plexus, push forward and send perforating branches through Bowman's mem- 

 brane and form on its surface a subepithelial plexus, the minute fibres of which pass 

 in a radial manner toward the center of the cornea. From this plexus fine fibrils 

 ascend between the epithelial cells, and end either as varicose fibrils, or in connection 

 with special end-bulbs (the intraepithelial plexiis}. In the substantia propria the 

 branches from the fundamental plexus, after forming complex secondary plexuses, 

 end as naked fibrillae between the lamellae, probably in close connection with the 

 corneal corpuscles. 



Practical Considerations. The external or fibrous covering of the eyeball 

 consists of the sclera and cornea, and is the protective covering. The posterior five- 

 sixths is made up of sclera, which in some animals becomes cartilaginous or even 

 bony. In the human eye the average normal tension within the globe is equivalent 

 to a column of mercury 26 mm. high. Excessive intraocular tension occurs under 

 pathological conditions (glaucoma) and may reach 70 mm. or more. The more 

 delicate structures then suffer severely and unless the pressure is relieved they are 

 functionally destroyed. The sclera is thickest and strongest posteriorly and grad- 

 ually grows thinner as it passes forward. Immediately behind the insertions of the 

 recti muscles it is thinnest (.4 mm.). Here bulging is most likely to occur from 

 internal pressure (anterior scleral or ciliary staphyloma), or pus within to burrow 

 through. In front of this zone it is reinforced by expansions from the insertions of 

 the muscles, and would seem therefore to be stronger, although it is in this region, 

 just back of the margin of the cornea, that ruptures are most likely to occur from 

 external violence. 



Ruptures of the sclera occur close to within 3 mm. of the corneal margin and 

 concentric with it, because in most cases, as Fuchs points out, the application of the 

 force does not lie in the centre of the cornea, but in the sclera below and to the outer 

 side of the cornea. The greatest expansion of the sclera takes place in its upper half 

 near the margin of the cornea, at which place, therefore, the sclera ruptures. 



This region is the so-called dangerous zone of the eyeball, because the iris and 

 ciliary body correspond to it, and in wounds involving these structures, sympathetic 

 ophthalmia frequently results, often leading to destruction of both eyes. Besides the 

 anterior staphylomata of the sclera, we may have the equatorial and the posterior. 

 The equatorial develops at the spots where the venae vorticosae penetrate and thus 

 weaken the sclera about the equator of the globe. 



The posterior is assumed to be the result of a congenital weakness of the sclera. 

 The anterior or equatorial can be seen or palpated, while the posterior is recognized 

 only by demonstrating the existence of a high degree of short-sightedness, which is 

 due to an increase of the sagittal axis of the eyeball. 



Rupture of the sclera is usually the result of a blow on the eye. The ciliary body 

 and anterior portion of the choroid are frequently forced into the wound, the vitreous 

 and aqueous chambers contain blood, while the lens may find its way through the 

 rent and lie under the conjunctiva, which may or may not be torn. Rarely the rup- 

 ture will be in the posterior portion of the globe. 



Congenital opacities of the -cornea may occur and may be complete or partial. 

 In some of the cases reported of the complete variety the anterior elastic lamina 

 was absent, and the anterior layers of the stroma were not laminated as usual, but 

 crossed each other, and among them were found blood-vessels. The partial 

 varieties may consist of a dense white opaque ring at the margin of the cornea, 

 as though the sclera had extended into the cornea, or they may resemble an arcus 

 senilis in which a perfectly clear strip of cornea divides the opaque line from the 

 margin of the sclera. 



The cornea in health is transparent, and almost all pathological lesions render it 

 opaque. It is the most exposed and therefore the most frequently injured part of the 

 eye. Wounds of the cornea heal readily under favorable circumstances, showing that 

 its nutrition is good, although there are no vessels in it, except within 1-2 mm. of 

 its margin. When the cornea is inflamed, however, new vessels may form from 

 those at the margin and extend a variable distance inward. Under the influence of 



