UiSSECTION OF THE DOG 191 



Ibis. — The iris is a muscular diaphragm placed immediately in front of the 

 crystalline lens. About its centre is an almost circular opening, the pupil 

 (pupilla) , which varies in size during Uf e to regulate the amount of light entering 

 the eye. The variations in the size of the pupil are governed by muscular 

 fibres within the substance of the iris. Some of these are disposed in a circular 

 manner and act as a sphincter (m. sphincter pupillae). 



The iris has two margins. One (margo pupiUaris) circumscribes the pupil. 

 The other (margo ciUaris) is connected with the ciliary body and, by the pectinate 

 ligament of the iris, mth the posterior lamina of the cornea. 



Dissection. — The retina may be demonstrated in several ways. By carefully 

 removing the vitreous body from the eyeballs which have been sectioned 

 sagittally and transversely, a good view of the retina from within can be 

 obtained. It may also be studied in the specimen in which the sclera 

 and cornea have been removed for the exhibition of the vascular tunic. 

 In this specimen the choroid, ciliary body, and iris must be cautiously 

 removed bit by bit. 



Retina. — The pigmentary (outermost) layer of the retina is so closely 

 adherent to the deep face of the choroid that it is removed mth this tunic. 

 The rest of the retina consists of nervous and connective tissue. Within the 

 nervous layers of the retina the fibres of the optic nerve have their origin, a 

 circumstance which accounts for the gradual increase in the thickness of the 

 membrane as its point of connection with the nerve is approached. 



At an indistinct line, the ora serrata, close to the margin of the ciHary body, 

 the nerve elements cease, and the retina suddenly becomes thinner. Prom 

 this line the membrane is continued to the ciliary body as the ciliary part 

 of the retina (pars ciUaris retinae). 



When the retina is viewed from within, the entrance of the optic nerve is 

 distinguished as a sUght, round or triangular elevation, the optic papilla (papilla 

 nervi optici), lying a Uttle below and lateral to the posterior pole of the eyeball. 

 Three or four small arteries, branches of the central artery of the retma, with 

 their satelhte veins, radiate from the papilla. 



Dissection. — To obtain the isolated vitreous body and crystalline lens, 

 the method suggested by Anderson Stuart should be followed. An 

 eyeball is allowed to lie untouched for some time — one to three days 

 according to season. An incision is then made along the equator through 

 the three tunics, and the two halves of the eyeball gently separated. If 

 this is properly done, the vitreous body and lens may be allowed to drop 

 entire into water, or still better into a stain in which they may remain 

 for some minutes. If stained, the specimen should be well washed 

 before examination. 



CoEPiTS VITEEXJM. — The vitreous body is a transparent, jelly-like substance 



