THE SCLEEOTIC. 589 



thick stem of the optic nerve behind. The recti and obliqui muscles 

 closely smTound the greater i^art of the eyeball, and are capable of 

 changing its position within certain limits : the lids, with the plica 

 semilunaris and caruncle, are in contact with its covering of conjunctiva 

 in fi'ont ; and behind it is supported by a quantity of loose fat and 

 connective tissue. 



The eyeball is composed of segments of two spheres, of which the 

 anterior is the smaller and more prominent : the segment of the larger 

 posterior opaque sphere corresponds with the limit of the sclerotic 

 coat, and the translucent portion of the smaller sphere with that of the 

 cornea. 



From before backwards the ball measures about nine-tenths of an 

 inch, and its transverse diameter exceeds this measurement by about a 

 line. 



Except when directed towards near objects, the axes of the eyes arc 

 nearly pai'allel ; the optic nerves, on the contrary, diverge considerably 

 from one another, and each nerve enters the corresponding eye about a 

 tenth of an inch to the inner or nasal side of the axis of the globe. 



The eyeball consists of several concentric coats, and of certain fluid 

 and solid parts contained within them. The coats or membranes are 

 three in number, viz.: an external fibrous covering, named sclerotic and 

 cornea ; a middle vascular, pigmented, and in part also muscular mem- 

 brane, the clioroid and the iris : and an internal nervous stratum, the 

 retina. The enclosed refracting media, three in number, are the aqueous 

 Jmmour,thQ vitreous bod;/, and the k)is with its capsule. 



Around the eyeball there is an adventitious tunic of fascia, tunira 

 vaginaJis ocuJi, or capsule of Tenon, which is perforated by the tendons 

 of the recti and obliqui muscles, and connected with the sclerotic by 

 merely the most delicate connective tissue. This capsule, which in 

 reality consists of two membranous layers lined by flattened epithelioid 

 cells, and enclosing a lymph space, separates the eyeball from the 

 orbital fat, and enables it to glide freely in its movements. 



THE SCLEROTIC COAT. 



The sclerotic (cornea opaca), the tunic of the eye on which the main- 

 tenance of the form of the organ chiefly depends, is a strong, opaque, 

 unyielding fibrous structure. It extends over about five-sixths of the 

 eyeball (fig. 399, 2), joining in front with the cornea. The outer surface 

 is white and smooth, except where the tendons of the recti and obliqui 

 muscles are inserted into it. The inner surface is a light brown colour, 

 and rough from the presence of a delicate connective tissue {membrana 

 fusca), which unites it with the choroid coat, and through which 

 branches of the ciliary vessels and nerves cross obliquely. The sclerotic 

 is thickest at the back part of the eye, and thinnest at about a quarter 

 of an inch from the cornea : at the junction with the latter, it is again 

 somewhat thickened. The optic nerve pierces this coat about one- 

 tenth of an inch internal to the axis of the ball, and the opening is 

 somewhat smaller at the inner than at the outer surface of the coat. 

 The outer fibrous sheath of the nerve blends with the sclerotic at the 

 margin of the aperture : in consequence of this arrangement, when the 

 nerve is cut off close to the eyeball, the funiculi seem to enter by a 

 gi'oup of pores ; and to the part of the sclerotic thus perforated the 



