PHYSIOLOGICAL ANATOMY OF THE EYEBALL. 77! 



The fibres are slightly wavy, and arranged in flattened bands, which are alternately longi- 

 tudinal and transverse, giving the membrane a lamellated appearance, although it cannot 

 be separated into distinct layers. Mixed with these bands of connective-tissue fibres, are 

 numerous small fibres of elastic tissue. The vessels of the sclerotic are scanty. They are 

 derived from the ciliary vessels and the vessels of the muscles of the eyeball. The tissue 

 of the sclerotic yields gelatine on boiling. 



Cornea. The cornea is the transparent membrane which covers about the anterior 

 sixth of the globe of the eye. As before remarked, this is the most prominent portion of 

 the eyeball. It is in the form of a segment of a sphere attached by its borders to the 

 segment of the larger sphere formed by the sclerotic. The thickness of the cornea is 

 about jV f an m h m i ts central portion, and about fa of an inch near its periphery. Its 

 substance is composed of transparent fibres, arranged in incomplete layers, something like 

 the layers of the sclerotic. It yields chondrine, instead of gelatine, on boiling. 



Upon the external, or convex surface of the cornea, are several layers of delicate, 

 transparent, nucleated epithelium. The most superficial cells are flattened, the middle 

 cells are rounded, and the deepest cells are elongated and arranged perpendicularly. 

 These cells become slightly opaque and whitish after death. Just beneath the epithelial 

 covering of the cornea, is a very thin, transparent membrane, described by Bowman 

 under the name of the " anterior elastic lamella." This membrane, with its cells, is a 

 continuation of the conjunctiva. 



The proper corneal membrane is composed of excessively pale, flattened bundles of 

 fibres, interlacing with each other in every direction. Their arrangement is lamellated, 

 although they cannot be separated into complete and distinct layers. Between the bun- 

 dles of fibres, lie a great number of stellate, anastomosing, connective-tissue corpuscles. 

 In these cells and in the intervals between the fibres, there is a considerable quantity of 

 transparent liquid. The fibres constituting the substance of the cornea are continuous 

 with the fibrous structure of the sclerotic, from which they cannot be separated by 

 maceration. At the margin of the cornea, the opaque fibres of the sclerotic abruptly 

 become transparent. The corneal substance is very tough, and it will resist a pressure 

 sufficient to rupture the sclerotic. 



Upon the posterior, or concave surface of the cornea, is the membrane of Descemet, 

 or of Demours. This is elastic, transparent, structureless, rather loosely attached, and 

 covered with a single layer of regularly polygonal, nucleated epithelium. At the circum- 

 ference of the cornea, a portion of this membrane passes to the anterior surface of the 

 iris, in the form of numerous processes which constitute the ligamentum iridis pectinatum, 

 a portion passes into the substance of the ciliary muscle, and a portion is continuous 

 with the fibrous structure of the sclerotic. 



In the adult, the cornea is almost without blood-vessels, but in foetal life it presents 

 a rich plexus extending nearly to the centre. These disappear, however, before birth, 

 leaving a very few delicate, looped vessels at the extreme edge. 



A great deal of anatomical interest has lately been attached to the cornea, from 

 researches showing the termination of the fine nerve-fibres in the nuclei of the posterior 

 layer of the epithelium of its convex surface and the investigation of the " lymph-spaces" 

 by the use of certain reagents, the demonstration of the so-called u wandering cells," 

 etc., points that we do not propose to consider. It is well known that the surface of the 

 cornea is exquisitely sensitive. 



Choroid Coat. Calling the sclerotic and the cornea the first coat of the eyeball, the 

 second is the choroid, with the ciliary processes, the ciliary muscle, and the iris. This 

 was called by the older anatomists the uvea, a name which was later applied, sometimes 

 to the entire iris, and sometimes to its posterior, or pigmentary layer. We shall describe, 

 however, the choroid and ciliary processes together as the second coat, and then take up 

 the ciliary muscle and the iris. 



