420 



CHORDATE ANATOMY 



vibrate freely. Temporary deafness occurs when the auditory tube is 

 closed because of inflammation caused by a cold. Functionally, however, 

 the most important elements of the middle ear are the three ear bones. 



The Internal Ear. The membranous sac or true organ of equilibration 

 and hearing is so complex that it is frequently called the membranous 

 labyrinth. The otic bone in which it is embedded takes on its shape and 

 is equally complex, and hence is known as the bony labyrinth. Except in 

 the region where the membranous labyrinth is attached to the bone, it is 

 surrounded by a cavity filled with the periljnnph. The sac itself is filled 

 with endol5miph. (Fig. 368) 



-COCHLEAR NERVE 

 -COCHLEAR DUCT 



EUSTACHIAN TUBE 



Fig. 369. — The human ear. "Sound waves" impinge upon the drum and are 

 carried by the three ear bones to the fenestra vestibuli and thus to the scala vestibuli 

 and the cochlear duct. (Redrawn from W. H. Howell after Czermak.) In life the apex 

 of the cochlea lies toward the Eustachian tube. In the Figure, for diagrammatic pur- 

 poses, it is rotated i8o degrees. 



The Organ of Equilibration. The function of equilibration in man is 

 served, as in lower vertebrates, by the utriculus and the three semicircular 

 canals connected with it. All three are hollow membranous canals Hned 

 by columnar epithelium and loosely attached by connective tissue to the 

 periosteum of the otic bone. Connected at both ends with the utriculus, 

 each canal swells at one end into an ampulla. On one side of each ampulla, 

 an elongated cluster of hair-cells forms a crista, which is innervated by a 

 branch of the vestibular nerve. 



The elongated hairs of the sensory cells of the crista extend into the 

 endolymph, the movements of which are communicated to the hairs, and 

 thus indirectly to the nerves connected with the hair-cells. Similar 

 patches of hair-cells, the maculae, with shorter hairs, occur on the sides 

 of the utriculus and sacculus and, like the cristae, have a static function. 



