552 HEARING. 



rinth by the immediate action of the air upon the membrane of 

 the fenestra rotunda, which Scarpa calls the secondary membrane 

 of the tympanum. 



The labyrinth, so called on account of its complicated canals, 

 s the internal part of the organs of hearing, and is hewn out of 

 the hardest portion of the temporal bone. It contains the sub- 

 stance of the auditory nerves, variously spread as layers and fibres 

 in a gelatinous water. Its parts are the three semicircular 

 canals, the vestibule, and the cochlea. 



The semicircular canals, two of which are vertical, and the 

 third nearly horizontal, contain similar membranous canals, each 

 of which has a swelling at its extremity. These canals terminate 

 by their extremities in the vestibule or central cavity. The 

 cochlea, in part osseous and partly membranous, winds round a 

 conical axis, in a spiral which makes two turns and a half and 

 which diminishes so that the cochlea approaches to the globular 

 form. One of its two gyrations terminates at the fenestra rotunda, 

 which communicates with the cavity of the tympanum ; the other 

 proceeds to the vestibule, which itself communicates with the same 

 cavity by means of the fenestra ovalis. 



" The vestibule and semicircular canals loosely contain very 

 delicate membranous bags, discovered by the celebrated Scarpa : 

 viz., two sacs which lie in the vestibule, and three semicircular 

 ducts in the canals of the same name." 



" These sacs, as well as the cavity of the cochlea, contain a very 

 limpid fluid, bearing the name of Cotugno, who showed it to be 

 absorbed by two canals, which are by him denominated aque- 

 ducts , and by Meckel diverticula p : the one arises from the vesti- 

 bule, the other from the inferior scala of the cochlea. 



" The portio mollis of the seventh" pair or acoustic nerve arises 

 from the fore-part of the floor of the fourth ventricle, is at first 

 soft, but soon becomes firmer and more fibrous, and, " having, to- 

 gether with the portio dura (which afterwards runs along the 

 aqueduct of Fallopius <i), entered the internal acoustic opening, 

 transmits its medullary filaments into the lower and cribriform 



" " Scarpa, Disquisitiones Anatomicce de Auditu et Olfactu, tab. iv. fig. 5. 

 tab. vii. fig. 3." 



" Cotunni, De Aquceductibus aims humance. Neap. 1761. 4to." 



p " Ph. Fr. Meckel, De Labyrinthi auris contentis. Argent. 1777. 4(0-." 



q " Fallopius, Observ. Anat. p. 27. b. sq. Venet. 1561. 8vo." 



