THE FROG AND VERTEBRATES IN GENERAL 217 



sound and those of position, or equilibrium. In the higher Verte- 

 brates, the ear consists of three parts : (1) the outer ear, which 

 is modified for collecting the sound waves ; (2) the middle ear, 

 which receives the waves thus collected and conveys them to 

 (3) the inner ear, which is the essential part and contains the 

 nerve tissue. In the Frog, however, the outer ear is lacking, and 

 the sound waves come first into contact with the tympanic mem- 

 brane. (W. f. 148.) 



Considering, first, the structure of the middle ear, it should be 

 noted that it is an accessory structure by which the sounds are 

 conveyed from the external environment to the inner, essential 

 sensory tissue of the ear. The middle ear of the Frog, which lies 

 near the outer surface of the body, begins with the tympanic mem- 

 brane, as noted above. The latter forms the closing membrane of 

 a tube which constitutes the cavity of the middle ear, and which 

 communicates with the mouth cavity by means of the Eustachian 

 ture. The openings of the Eustachian tubes in the mouth have 

 been previously noted. The tympanic membrane is of such a 

 nature that the impinging sound waves cause it to vibrate, and these 

 tympanic vibrations are then conveyed through the cavity of the 

 middle ear and into the inner ear by a rod, the columella, one end 

 of which is attached to the tympanic membrane, and the other 

 end to a portion of the inner ear. In the higher Vertebrates, the 

 columella is replaced by three bones in the middle ear, which are 

 termed, in accordance with their shape, the malleus, incus, 

 and stapes, or commonly, the hammer, anvil, and stirrup. Col- 

 lectively they constitute the auditory ossicles. 



The inner ear lies in a specialized bony cavity of the skull, 

 known as the auditory capsule, and is surrounded by the liquid 

 perilymph which fills the auditory capsule. The inner ear con- 

 sists of a very complicated structure, known as the memrranous 

 laryrinth, which contains the essential sensory tissue. The 

 membranous labyrinth is composed of a large upper portion, the 

 utriculus, which is concerned with the sense of position, and a 

 small portion lying below, the sacculus, which is concerned with 

 the sense of hearing. The sacculus has an irregular, bag-like shape, 

 is filled with a fluid known as the endolymph, and contains the 

 nerve endings. The vibrations of the columella cause vibrations 

 in the endolymph and these, in some way, influence the sensory 

 cells. The impulses thus received are conveyed to the brain by 



