THE SENSE ORGANS 



599 



the meatus, is the concha. The opening of the meatus is guarded by two 

 processes, a ventral tragus next the cheek and a dorsal antitragus opposite. 

 The incurved outer rim of the auricle is the helix. The antihelix is a 

 smaller ridge which bounds the concha dorsally. The walls of the meatus 

 are supported laterally by fibro-cartilage and medially by bone and lined 

 by skin. The meatus is beset with hairs, and contains many tubular 

 glands, which secrete the ear-wax. 



The Middle Ear. A tympanic membrane or ear-drum separates the 

 external meatus from the tympanic cavity or middle ear, within which lie 

 the ear bones. Leading from the tympanic cavity to the pharynx, is the 

 auditory or Eustachian tube, which serves to equalize the atmospheric 



-COCHLEAR NERVE 



"~"'~^'' i#j55*^-COCHLEAR DUCT 



EUSTACHIAN TUBE 



Fig. 496. — 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.) 



pressure on the two sides of the tympanic membrane, so that this may 

 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 iilled with the perilymph. The sac itself is filled 

 with endolymph. (Fig. 495) 



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 lined 



