THE SENSE OF SIGHT. 137 



THE SENSE OF SIGHT. 



The Eyeball. The eyeball, or organ of vision, is situated within the 

 orbital cavity, and loosely held in position by the fibrous capsule of Tenon. 

 It rests upon a cushion of fat, which never disappears, except in cases of 

 extreme starvation ; it is protected from injury by the bony orbital walls, the 

 lids and lashes, and is so situated as to permit an extensive range of vision. 



Blood vessels and Nerves. The structures of the eyeball are supplied 

 with blood by the ciliary arteries, which pierce the posterior surface around 

 the optic nerve. 



The Ciliary or Ophthalmic ganglion, about the size of a pin's head, 

 situated in the posterior portion of the orbital cavity, receives filaments of 

 communication from the trifacial or 5th nerve, the motor oculi or 3d nerve, 

 and the sympathetic. From its anterior portion are given off the ciliary 

 nerves, which enter the ball posteriorly and are distributed to the structures 

 of which it is composed. 



Structure. The form of the eyeball is that of a sphere ; it is about one 

 inch in the transverse diameter, and a little longer in the antero-posterior 

 diameter, on account of its having the segment of a smaller sphere inserted 

 into the anterior surface. 



The Sclerotic and Cornea together form the external coat of the eye ; 

 the former covering the posterior J, the latter, the anterior \. The 

 sclerotic is a dense, opaque, fibrous membrane, varying in thickness from 

 the JQ- to the -^ of an inch ; it is composed of connective tissue and is 

 slightly vascular. Posteriorly it is continuous with the sheath of the optic 

 nerve, and is pierced by that nerve, as well as by the ciliary vessels and 

 nerves ; anteriorly its fibres become quite pale, and after passing into the 

 cornea, transparent. It is a protective covering, and gives attachment to 

 the tendons of the muscles by which the eyeball is moved. 



The Cornea is a non- vascular, transparent membrane, composed for the 

 most part of connective tissue in which are contained stellate corpuscles 

 filled with a clear fluid. It is covered anteriorly by the basement mem- 

 brane of the conjunctiva, upon which rests several layers of epithelial cells ; 

 posteriorly it is lined by the membrane of Descemet, which is reflected on 

 to the anterior surface of the iris. 



The Choroid, the Iris, the Ciliary Muscle and Ciliary Processes, 



together constitute the middle coat of the eye. 



The Choroid coat, about the ^ of an inch in thickness, is the vascular 

 tunic ; it is connected with the sclerotic by the lamina fusca. From within 

 outward we distinguish the following layers : 

 J 



