THE FROG. 



19 



FIG. XVIII. The outer half of an eye, bisected at right angles to tho plaiie of the 

 optic nerve. 



FIG. XIX. The inner half of the same. 



In both figures the internal surface is drawn. All x 31. 



FIG. XX. Transverse section of the head, immediately behind the occiput, especially 

 designed to show the relations of the auditory organ. 



On the left side all structures, excluding the post auditory cranial nerves, have been 

 removed to the level of the columella auris and hind wall of the skull ; on the right the 

 auditory capsule has been opened up to the level of the vinth cranial nerve. x 3. 



FIG. XXI. The membrana tympani, seen from without. 



The dotted lines indicate the course of the mucous membrane lining the Eustachiau 

 recess, and in both this and the preceding figure, the tympanic auuulus is indicated in 

 black, x 3. 



FIG. XXII. The membranous labyrinth of the left side, seen from without, after 

 removal from the body. 



Holding the auditory capsule between the finger and thumb, the bone was sliced away 

 piecemeal with a small scalpel. x 6. 



FIG. XXIII. The same, seen from within, a portion of the auditory nerve having been 

 removed with it. x 6. 



The structural details of this organ have recently been fully worked out by Ketzius, in 

 his classical monograph, "Das Gehororgan der Wirbelthiere," vol. i., Stockholm, 1881. 

 Figtircs reproduced in Wiedersheim and Ecker (24). 



a.c. Anterior commissure. 



am'. Ampulla of the anterior semicircular canal. 



am''. Ampulla of the posterior semicircular canal. 



am"'. Ainpulla of the horizontal semicircular 



canal. 



aij.c. Anterior (aqueous) chamber. 



a</.s. Aqueduct of Sylvius (Iter). 



a.s.c. Anterior-vertical semicircular canal. 



at. First vertebra. 



a.tij. Annulus tyrnpaiiicus. 



ax. Auditory capsule. 



b.jil. Brachial plexus. 



b.n. Brachial enlargement, 



c. Cornea. 



c'. Commissure between 7th and Oth cranial 



nerves. 



c.u. 



clt. 



cd. 



//. 



c./i. 



did. 



cj. 



'.//. 



c.(//. 



il./n. 



du. 



e.l. 



el'. 



Commissure between palatine and maxillary 



nerves. 



Columella auris. 

 Cerebellum. 



Occipital condyle (tig. 111. cardiac nerve). 

 Optic chiasma. 

 Cerebral hemisphere. 

 Choroid. 

 Conjunctiva. 

 Choroid plexus. 

 Chorda tympaui. 

 Dura junior. 

 Duodenum. 

 Upper eyelid. 

 Lower eyelid. 

 Tendon of the same. 



