THE EYE. 347 



contain occasional nuclei. From this point they again ex- 

 pand into broader sheets, which, after surrounding the outer 

 nuclei, are united to form the membrana limitans externa. 

 This membrane lies just at the base of the rods and cones, and 

 it is provided with numerous holes, through which those struc- 

 tures pass ; from its outer surface fibres extend up between 

 the rods and cones to form supporting sheaths. 



The blood-vessels of the retina come from the arteria cen- 

 tralis retince, which usually divides into two or more branches 

 at the entrance of the optic nerve ; these vessels lie in the layer 

 of nerve-fibres, and, arching above and below the macula, give 

 off numerous fine branches, from which capillaries penetrate 

 as far as the inner nuclear layer. The larger retinal vessels are 

 surrounded by lymph- spaces, which probably unite with those 

 of the optic nerve. 



At the periphery the retina becomes much thinned, and at 

 the ora serrata the nervous elements are discontinued, the con- 

 nective tissue alone being prolonged over the ciliary body to 

 its anterior angle, thus forming the pars ciliaris retinae. 



This membrane consists of long cylindrical cells of varying 

 shapes ; they rest on the pigment and are covered by a thin 

 stratum, which sends processes between them and seems to be 

 a prolongation of the membrana limitans interna of the retina. 



It is very difficult to prepare good sections of the retina, 

 but the following plan is recommended : enucleate with care 

 the eye of a frog or some small animal, and immediately sus- 

 pend it in a well-stoppered bottle containing a small bit of 

 solid osmic acid ; when sufficiently hard the posterior portion 

 of the eye can be cut in pieces and sections made by imbed- 

 ding or holding between pieces of liver. 



Another method is to place the eye unopened in Muller' s 

 fluid for some two weeks, frequently changing the fluid ; af- 

 terward harden in alcohol. Sections may then be made in the 

 same manner as before. 



To obtain the separate constituents, place a fresh retina in 

 a T V per cent, aqueous solution of osmic acid for fourteen days, 

 then in glycerine for seventeen days ; after this, place a small 

 piece on a slide in glycerine, with the cover-glass so arranged 

 that no pressure is made upon the specimen ; now tap gently 

 on the centre of the glass until the motion of the fluid causes 

 the retina to fall apart. 



