192 THE FROG CHAP. XI 



under the low and then under the high power of the micro- 

 scope. Note : 



1. The cartilaginous sclerotic. 



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



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

 notice the 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. (p. 556). 



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- 

 columella (Fig. 10). 



The essential part of the auditory organ (membranous 

 labyrinth) is very small in the frog, and can be more satis- 

 factorily 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 (p. 479), 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, etc., 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. 



