192 THE FROG CHAP, xi 



2. The choroid, enclosing pigment-cells and blood-vessels. 



3. The retina (Fig. 58), composed of a number of layers : notice 

 \hz pigment epithelium, the rod- and cone-layer, and the various other 

 layers of the retina, the innermost of which is composed of nerve-fibres 

 continuous with the optic nerve. 



Sketch. 



The anatomy of the eye can be more easily made out by dissecting 

 that of an ox or sheep, which is essentially similar to that of the frog, 

 and directions for the examination of which will be given in Part II. 



III. Auditory organ. 



Notice again the tympanic membrane and tympanic ring, and then 

 carefully cut away the former so as to expose the tympanic cavity. 

 Observe the Eustachian tube, the fenestra ovalis, and the relations of 

 the stapes, columella, and extra-cohimella (Fig. 10). 



The essential part of the auditory organ (membranous labyrinth] is 

 very small in the frog, and can be more satisfactorily studied in a good- 

 sized fish (e.g., dogfish or cod). Directions for the preparation of the 

 membranous labyrinth of the former will be given at a later stage, 

 but if you have sufficient time and patience to dissect it out in the frog, 

 proceed as follows : 



Place the head of a large frog in nitric acid (about 10 per cent.) until 

 the bone is dissolved. Wash well in water so as to remove the acid, 

 and dissect away the muscle, c., from the auditory capsule until the 

 latter is thoroughly exposed. Then with a sharp scalpel slice away the 

 roof of the capsule until the cavity it contains is seen. Proceed now 

 with great caution, removing the cartilage and decalcified bone, bit by 

 bit, until the cavity is sufficiently enlarged to bring the membranous 

 labyrinth into view (compare Figs. 10 and 59). Observe the utriculus, 

 sacculus, otoliths, and the three semicircular canals with their ampulla. 

 Sketch. 



