io8 PRACTICAL HISTOLOGY. 



cells. The nucleus usually does not contain pigment, though the rest of the cell and its 

 processes are loaded with fine granules (melanin) of a brown or black colour (PI. XXV., 

 Fig. 6). 



THE CILIARY MUSCLE. 







PREPARATION. Divide the eyeball of an ox or sheep, with a sharp razor, transversely, 

 half an inch behind the circumference of the cornea, and remove the lens, but be careful to 

 retain the choroid and iris. Harden the anterior half of the eyeball in the chromic acid and 

 spirit fluid for eight days. Make horizontal sections, to include the sclerotic, the cornea, and 

 the iris. Stain a section with picrocarmine or logwood and mount it in Farrant's solution or 

 dammar. 



EXAMINATION (L). Observe the cornea in the front part continuous with the sclerotic 

 behind. At the line of junction notice the section of the pigmented iris projecting inwards. 

 Trace the choroid posteriorly, lining the sclerotic. Observe the fibres of the ciliary muscle 

 arising at the junction of the sclerotic and cornea, and passing backwards over and outside 

 the choroid, into which it is inserted. Sections of the ciliary processes may be seen. Trace 

 the membrane of Descemet of the cornea backwards, and notice that at the junction of the 

 cornea and sclerotic it splits into fine transparent bundles, some of which curve round towards 

 the iris, whilst others spread out like a fan over the ciliary processes, thus forming the liga- 

 mentum pectinatum iridis. At the junction of the cornea and sclerotic notice a small aperture. 

 This is a section of the venous sinus, or canal of Schlemm (PI. XXVI., Fig. 4). 



RETINA. 



PREPARATION. Mtiller's Fluid and Spirit. It is desirable to use the retinae of several 

 animals, because each shows some special feature. 



(a) With a sharp razor divide the eyeball of a pig, cat, and ox transversely, and place the 

 posterior halves in a mixture of three parts of M tiller's fluid and one of spirit, and keep the 

 preparations in the dark and in a cool place for a week or ten days. Make vertical sections 

 of each of the above in the ordinary way, by means of a freezing microtome, after the 

 tissues have been soaked in syrup and then in gum, as described at p. xxxix. This is a most 

 important precaution to ensure good preparations. Make the sections through the sclerotic, 

 choroid, and retina, though the retina is very apt to separate from the other coats. Stain a 

 section of each, with either logwood or picrocarmine. Mount them in Farrant's solution. 

 This method shows the nerve-elements best. 



(#) Chromic Acid and Spirit Mixture. This may also be used, and shows the connective- 

 tissue elements best ; besides, it does not make the retina quite so brittle as the Miiller's 

 fluid. 



(c) Osmic Acid. Place the retina of a frog and that of a fish e.g. cod in a few cubic centi- 

 metres of a quarter per cent, solution of osmic acid for three or four hours. The retinas are 

 rapidly blackened and ' fixed.' Soak the retinae for several hours in water to get rid of the 

 surplus osmic acid, and tease a small part of each in a drop of glycerine, and cover. 



Vertical Section of the Retina of a Cat or Pig. EXAMINATION (L). Observe from without 

 inwards : 



