268 Mammalia. 



inside of tlie eye through the vitreous body, the surface of the 

 Hetina, as long as it remains properly supported by this 'body/ 

 will be seen to be quite smooth, except at one spot, rather to 

 the side of the axis of the eyeball, where the optic nerve 

 expands, and the central artery of the Retina enters. 



The Yitreous body in its Hyaloid membrane should now 

 be removed in order to see the inner surface of the Nervous 

 Tunic or Retina : i.e. the delicate, almost pulpy membrane, 

 which contains the terminal part of the optic nerve, lies 

 within the Choroid coat, rests on the Hyaloid membrane of 

 the Vitreous humor, and, extends forwards nearly to the 

 outer edge of the ciliary processes of the Choroid, where it 

 ends in a finely indented border viz. the ova serrata. 



"We do not find in the Eye of the Ox, nor indeed in the 

 Eye of any Mammal, except that of Man and of the higher 

 Quadrumana, the yellow elliptical spot in the axis of the eye- 

 ball known as the* macula lutea'or yellow spot of Sommerring. 



The Retina is easily separable from the investing membrane 

 next external to it, viz. the Choroid coat ; and when this is 

 detached, the outer surface will be seen to be rough or slightly 

 flocculent, from the fine shreds or fragments of a structure at 

 first called JacoVs Membrane, but which is now generally 

 recognised as the columnar layer of the Retina. 



To prepare the Retina for Microscopic investigation, the 

 Student should take a perfectly fresh specimen and harden it 

 by immersing it in Miiller's fluid (consisting of 2^ parts of 

 Potassium Bichromate, 1 part of Eodium Sulphate, and 100 

 parts of Distilled Water) ; then he should make a mixture of 

 bee's- wax and olive oil (equal parts), and pour these substances 

 when melted (at as low a temperature as possible) round the 

 piece of Retina from which the sections are to be cut ; or 

 place the piece of Retina (if flat), sandwich- fashion, between 

 flat plates made of these same substances. By this means 



