THE EYEBALL. THE SCLEFxOTIC COAT. 711 



The eyeball, when viewed in profile, is found to be 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 are nearly 

 parallel ; 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 is composed of several investing membranes, concentrically 

 arranged, and of certain fluid and solid paits contained within them. The 

 membranes are three in number, with the following designations and general 

 structure : An external fibrous covering, named sclerotic and cornea ; a 

 middle vascular, pigmentary, and in part also muscular membrane, the choroid 

 and the iris ; and an iuternal nervous stratum, the retina. The enclosed 

 refracting media, three in number, are the aqueous humour, the vitreous body, 

 and the lens with its capsule. 



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

 vaginalis oculij 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 separates the eye-ball from the 

 orbital fat, and enables it to glide freely in its movements. (See, for details, 

 Richet, Traite d'Anatomie Medico-Chirurgicale ; and O'Ferrall, in Dublin 

 Quart. Journ. Med. Science, July, 1841.) 



EXTERNAL COAT OF THE EYEBALL. 



The external investing membrane, which forms a complete covering for the 

 ball, consists of two parts of different appearance and structure. Of these 

 the hinder part, much the largest, is opaque and densely fibrous, and is 

 named the sclerotic coat, while the anterior smaller segment is transparent, 

 and is named the cornea. 



THE SCLEROTIC COAT. 



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

 nance of the form of the greater part of the organ chiefly depends, is a 

 strong, opaque, unyielding, fibrous structure. The membrane covers about 

 five-sixths of the eye-ball, and is pierced behind by the optic nerve. 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 of a light brown 

 colour, and rough from the presence of a delicate connective tissue (membrana 

 fusca), 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 

 cornea, 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 

 fibrous sheath of the nerve, together with the membranous processes which 

 separate the funiculi of its fibres, blend with the sclerotic at the margin of 

 the aperture : in consequence of this arrangement, when the neive is cut off 



