COATS OF THE EYHBALL. 521 



the caudal part of the sclerotic to make it transparent ; hold the 

 eye with the cornea toward a well-lighted window or a lamp flame. 

 The image of the window or flame will be seen on the caudal aspect 

 of the eyeball ; the image is real, and hence inverted like that 

 formed by a photographer's camera. Eaise or lower the eyeball, 

 and the image will be seen to move in the opposite direction. If the 

 eye of a large animal is used for this experiment, a piece must be 

 removed from the caudal part of the sclerotic on account of its 

 opacity. 



§ 1420. TunicsB oculi— Coats of the eye (Fig. 126).— For the 

 study of the remaining parts of the eye, a fresh one may be used, 

 but one hardened in alcohol is desirable, as such a one retains its 

 form and the various parts are less easily torn and displaced. To 

 harden an eye, cut a slit in the sclerotic at one side and place the 

 eye on absorbent cotton in 62 per cent, alcohol for a day ; then 

 remove to 95 per cent, for two days or more. 



Dissection. — With forceps and scissors make an incision from 

 about the middle of the cornea to near the optic nerve, taking care 

 to cut only through the wall. Connect the same two points by 

 another incision in such a way as to rempve a segment containing 

 one fourth or one fifth of the entire wall of the globe. In this seg- 

 ment the different tunics may be studied. 



§ 1421. Sclerotica et Cornea (Fig. 126).— Together these form 

 the ectal covering or framework of the eyeball. 



The sclerotic covers the caudal three fourths of the eyeball and 

 becomes thickened before merging into the cornea. This thicken- 

 ing has the appearance of a white band around the eyeball, and 

 for convenience may be called the Zona alba (Fig. 126, Z. a.) or 

 white zone. At the caudal margin of this zone are inserted the recti 

 muscles. Its width indicates the length of the plicce ciliares (§ 1422). 



In the ental wall of the sclerotic are many pigment cells {lamina 

 fused), giving it a dark appearance ; and on the line where it 

 merges into the cornea these pigment cells extend through to the 

 ectal wall of the sclerotic. The cornea completes the framework of 

 the eyeball cephalad. It is transparent and intermediate in thick- 

 ness between that of the white zone and the rest of the sclerotic. 



§ 1422. Ohoroidea (Fig. 126). — The choroid coat of the eye is 

 just entad of the sclerotic. It is a vascular coat, but contains also 

 much pigment, hence its dark appearance. AVith a tracer separate 

 the choroid and sclerotic as shown in Fig. 126. The choroid does 



