70 ANATOMY OF THE HEAD AND NECK. 



sheep. A number of them is desirable, and their dissection should lx 

 conducted in water. A soup-plate is a convenient dish for this pur- 

 pose. The muscles and the mucous membrane should all be remove " 

 and the eyeball left with the optic nerve alone attached to it. In ordei 

 to obtain a general idea of the different parts, the student will do well 

 to make botli a longitudinal and a transverse section of the eyeball, an< 

 this he will easily accomplish if he will first freeze the eye by immers- 

 ing it in a mixture of ice and salt. 



The ORGAN OF VISION is composed of certain parts essen- 

 tial to sight and of others requisite for their protection. 

 Its sentient constituent is an expansion of the optic nerve 

 (retina). A series of transparent parts (cornea, lens, vitre- 

 ous humor) bring the rays of light to a focus upon the 

 retina ; and a movable curtain (iris) regulates the extent of 

 their admission. To defend these structures, a dense tunic 

 (sclerotic) is arranged around them, and to absorb the 

 superabundant rays of light there is an internal tunic (cho- 

 roicl) provided with a layer of pigment. 



The SCLEROTIC TUNIC extends from the entrance of the 

 optic nerve to the cornea, and forms five-sixths of the sur- 

 face of the eyeball. The optic nerve perforates it a little 

 to the inner side of its centre ; a marked constrictioi 

 characterizes it just previous to its entrance; if the nerve 

 be drawn out, it will be seen that it leaves an orifice in the 

 sclerotic, tapering inward, and which at the bottom is pei 

 forated by minute holes, through which the fibres of th< 

 nerve penetrated; this spot has hence been called the lamim 

 cribrosa. The inner aspect of the sclerotic is flocculent witl 

 the ends of ruptured vessels and nerves ; externally it it 

 smooth except where muscles are attached, and it is thickest 

 at the posterior part. 



The CORNEA is a clear and diaphanous structure whicl 

 forms the anterior wall of the anterior chamber of the ey( 

 and admits the rays of light. It is circular in form, and, 

 like the crystal of a watch, is convex anteriorly and 

 cave posteriorly; at its circumference it blends with the 

 sclerotic by continuity of tissue ; its structure is laminated, 

 and it is covered superficially by the conjunctiva (p. 17) ; il 

 internal surface is lined by a structureless membrane, called 

 the membrane of Descemet or of Demours. In its health; 

 state the cornea is devoid of vessels ; after death it become 

 flaccid and opaque from infiltration of the aqueous humor. 



The vascular coat of the eyeball is internal to the scl< 

 rotic; it is a thin membrane made up of bloodvessels an< 

 pigment cells, divisible into three parts : a posterior poi 



