818 THE APPARATUS OF TEE SENSES. 



is transversal, and whose border, bevelled on the inner side, is closely united 

 to the circumference of the cornea. The substance of this membrane is 

 traversed by numerous vessels and nerves, and is not of the same thickness 

 throughout ; at the back, around the entrance of the optic nerve, it is 

 thickest ; it then diminishes gradually towards the larger axis of the organ, 

 and afterwards increases until it meets the cornea. 



Fig. 383. 



THEORETICAL SECTION OF THE HORSE'S EYE. 



a, Optic nerve; 6, Sclerotic j c, Choroid; d, Retina; e, Cornea; /, Iris; g, A, Ciliary 

 circle (or ligament) and processes given off by the choroid, though represented as 

 isolated from it, in order to indicate their limits more clearly ; t, Insertion of 

 the ciliary processes on the crystalline lens , /, Crystalline lens ; k, Crystal- 

 line capsule ; I, Vitreous body ; m, n, Anterior and posterior chambers ; o, The- 

 oretical indication of the membrane of the aqueous humour ; p, p, Tarsi ; q, q, 

 Fibrous membrane of the eyelids ; r, Elevator muscle of the upper eyelid ; s, s, 

 Orbicularis muscle of the eyelids ; t, t. Skin of the eyelids ; w. Conjunctiva ; v, 

 Epidermic layer of this membrane covering the cornea ; a?, Posterior rectus muscle ; 

 y, Superior rectus muscle , z, Inferior rectus muscle ; w, Fibrous sheath of the 

 orbit (or orbital membrane). 



STRUCTURE. The sclerotic is wholly composed of fasciculi of fibro- 

 cellular tissue interwoven in a very close manner, with some elastic fibres and 

 little masses of pigment, especially at its back part. Among these fasciculi 

 a large number pass from before to behind, and these are intersected by 

 others which are placed in a circular manner around the globe. The super- 

 ficial fibres are continuous with the neurilemma of the optic nerve. (The 

 optic nerve, at its entrance into the sclerotic, is very much constricted, 

 and passes through a funnel-shaped, porous mesh of fibrous tissue named the 

 lamina cribrosa, in the centre of which is a larger opening than the others, 

 for the passage of the arteria centralis retinae the porus opticus. The inner 



