THE EYE. 891 



serous membrane, consisting of a parietal and visceral layer. The latter invests 

 the posterior part of the globe from the ciliary margin of the cornea backward to 

 the entrance of the optic nerve, and is connected to it by very delicate connective 

 tissue : the former (parietal) lines the hollow in the fat in which the eyeball is 

 imbedded. Both layers are lined on their free surfaces by endothelial cells. The 

 cavity between them is continuous with the spaces between the different layers of 

 the sheath of the optic nerve that is to say, with the subarachnoidean between the 

 pia-matral and the arachnoid sheath, and the subdural between the arachnoid and 

 dural sheath and into it empty the lymphatic vessels of the sclerotic. The capsule 

 is pierced by the muscles of the eyeball near their insertion, and sends tubular 

 prolongations on them, which become continuous with the sheath of the muscles. 

 From the outer surface of these sheaths expansions, consisting of elastic fibres and 

 muscle-cells, are given off to the margin of the orbit, which serve to limit the 

 degree of contraction of the muscles. 1 



The eyeball is composed of segments of two spheres of different sizes. The 

 anterior segment is one of a small sphere, and forms about one-sixth of the eyeball. 

 It is more prominent than the posterior segment, which is one of a much larger 

 sphere, and forms about five-sixths of the globe. The segment of the larger sphere 

 is opaque, and formed by the sclerotic, the tunic of protection to the eyeball ; the 

 smaller sphere is transparent, and formed by the cornea. The axes of the eyeballs 

 are nearly parallel, and do not correspond to the axes of the orbits, which are 

 directed outward. The optic nerves follow the direction of the axes of the orbits, 

 and are therefore not parallel ; they enter the eyeball a little to their inner or nasal 

 side. The eyeball measures rather more in its transverse than in its antero- 

 posterior and vertical diameters, the former amounting to about an inch, the latter 

 to about nine-tenths of an inch. 



The eyeball is composed of several investing tunics, and of fluid and solid 

 refracting media, called humors. 



The tunics are three in number: 



1. Sclerotic and Cornea. 



2. Choroid, Iris, and Ciliary Processes. 



3. Retina. 



The refracting media, or humors, are also three : 



Aqueous. Crystalline (lens) and Capsule. Vitreous. 



The sclerotic and cornea form the external tunic of the eyeball ; they are 

 essentially fibrous in structure, the sclerotic being opaque, and forming the 

 posterior five-sixths of the globe ; the cornea, which forms the remaining sixth, 

 being transparent. 



The Sclerotic (ax).r t rio~. hard) (Fig. 527) has received its name from its extreme 

 density and hardness ; it is a firm, unyielding, fibrous membrane, serving to main- 

 tain the form of the globe. It is much thicker behind than in front. Its external 

 surface is of a white color, quite smooth, except at the points where the Recti and 

 Obliqui muscles are inserted into it, and covered, for part of its extent, by the 

 conjunctival membrane; hence the whiteness and brilliancy of the front of the 

 eyeball. Its inner surface is stained of a brown color, marked by grooves, in 

 which are lodged the ciliary nerves, and connected by an exceedingly fine cellular 

 tissue (lamina fusca) with the outer surface of the choroid. Behind, it is pierced 

 by the optic nerve a little to its inner or nasal side, and is continuous with the 

 fibrous sheath of the nerve, which is derived from the dura mater. At the point 

 where the optic nerve passes through the sclerotic this membrane forms a thin 

 cribriform lamina (the lamina cribrosa) ; the minute orifices in this layer serve 

 for the transmission of the nervous filaments, and the fibrous septa dividing them 

 from one another are continuous with the membranous processes which separate 

 the bundles of nerve-fibres. One of these openings, larger than the rest, occupies 



1 See a paper by Mr. C. B. Lockwood (Journal of Anatomy and Physioloyy, vol. xx., part i. p. 1). 



