78 SPECIAL SENSES. 



panum. The tympanum, by its delicate elasticity, augments 

 the vibrations, and transmits them to the internal ear, partly by 

 means of the little bones in the chamber, 

 which are disposed in such a manner that 

 the stapes exactly fits the oval aperture 

 (foramen ovafe) ; and partly by means of 

 the air which strikes the membrane cover- 

 ing the round aperture (foramen rotun- 

 dum), and produces vibrations there, cor- 



responding to those of the tympanum. 



Fir. 58. Horizontal After all these modifications, the sonorous 

 section through tube, vibrations arrive at last at the labyrinth 

 lamina, modiolus, and and the auditory nerve, which transmits 

 meatus internus. the impression to the brain. 



156. The mechanism of hearing is not so complicated 

 in all classes of animals, but is found to be more and more 

 simplified, as we descend the series. In birds, the middle 

 and internal parts of the ear are constructed on the same 

 plan as in mammals , but the outer ear no longer exists, and 

 the auditory passage, opening on a level with the surface of 

 the head behind the eyes, is surrounded only by a circle of 

 peculiarly formed feathers. The bones of the middle ear are 

 also less numerous, there being generally but one. 



[The owls have a large membranous cresceritic fold, provided 

 with tufts of short feathers, and which can be used as a valve. 

 The largest ear-conch is met with in the long-tufted hibou 

 (Strix otus) . A true chain of ossicles may be distinguished in 

 the tympanum, one of which is style-shaped and bony, while 

 the others remain in a cartilaginous state. The principal bone 

 represents the stajjes : its base forms an oval plate, which is 

 applied to the foramen ovale, and through this the sonorous 

 vibrations are transmitted to the aqueous fluid of the labyrinth. 

 Only one muscle can be detected for moving the ossicles, which 

 is thought to represent the laxator of the tympanum. The la- 

 byrinth consists of compact bony walls, surrounded by spongy 

 osseous tissue. The vestibule is small ; the semicircular canals 

 are large, and vary in size, being broad and elevated in rapa- 

 cious and passerine birds, and thick and depressed in the 

 grallse, gallinae, and palmipedes. The cochlea consists of a 

 slightly curved osseous cone. In the membranous sac of the 

 vestibule minute masses of crystallized phosphate of lime (oto- 

 itths) are found, as in mammals. T. \V.] 



