220 PRACTICAL NOTES ON HISTOLOGY. 



into distinct membranes. Before separating, the hexagonal pig- 

 ment cells which belong to the retina, but frequently adhere to 

 the inner surface of the choroid, must be entirely washed off 

 with a camel-hair pencil under spirit, and with the aid of a 

 dissecting-lens. 



The Musculature of the Iris. —The circular and radiating 

 plain muscular fibres of the iris may be demonstrated in the 

 albino rabbit. Cut the eye in half, and place the anterior part 

 in spirit for a day or more. Remove the lens and cut out a seg- 

 ment of the iris, including its whole width, from the pupillary 

 aperture to the ciliary processes of the choroid, and place in 

 dilute logwood. Do not let it be too deeply stained, then place in 

 water, pass through spirit and clove oil, and mount in dammar or 

 balsam, with the posterior surface uppermost. The thick ring of 

 the sphincter is easily seen in these preparations, and also the 

 interlacing bundles of plain muscular fibre, of which the dilator 

 is composed. 



The Iris.— It is useful to prepare the human iris, although 

 difficult on account of the uveal pigment, for the musculature is 

 somewhat different. Take an eye hardened in Miiller or bichro- 

 mate of potash two per cent., and subsequently in spirit. Cut out 

 a piece as before, and treat similarly, except that before staining 

 the pigment is brushed completely off the posterior surface with a 

 stiff camel's-hair brush under spirit. Examine the iris every now 

 and then with a low power, and see when all the pigment is 

 removed. Another way to show the Musculus Dilatator Pupillae 

 is to excise the iris and place it in strong acetic acid twelve hours, 

 or several days, in a weak solution. It is removed, brushed with 

 a soft brush, and carefully split with the point of a scalpel. Then 

 the anterior surface can be removed from the posterior surface of 

 the connective tissue and layer of blood-vessels. The remaining 

 layer of the iris so prepared is now stained with carmine, logwood, 

 or picro-carmine, and mounted in acidulated and diluted glycerine 

 if in the first instance. The following reagents are useful in making 

 the iris denser and more easy to split : — Chromic acid, o-qi per 

 cent. ; gold chloride, o'l per cent. ; palladium chloride. The 

 smooth muscle-fibres may be stained by putting them for some hours 



