492 



HISTORY OF THE HUMAN BODY 



of kinds of histological elements, which change their propor- 

 tionate size along- the course, being the smallest at the base and 

 the largest at the apex. The most important of these cellular 

 elements are certain elongated cells associated in pairs, the 

 rods of Corti, and two groups of cells with specialized terminal 

 organs, the outer and inner hair cells. The ventral portion of 

 the lagena, that is, the floor of the scala media beyond the 

 auditory area, is termed the basilar membrane, and the dorsal 

 portion (roof) is Reissner's membrane; these terms are, how- 

 ever, purely anatomical ones, expressing certain relationships 



canalis cochlearis. 



FIG. 134. Diagram of membranous labyrinth of human ear. [From 

 GEGENBAUR, after RETZIUS.] 



A, A, A, ampullae; U, utriculus; S, sacculus; E, ductus endolymphaticus; 

 ant, ext, post, the three semicircular canals. 



to the surrounding parts and are without morphological 

 significance. 



The membranous labyrinth as above described, that is, the 

 higher development of the auditory vesicle of the embryo, be- 

 comes surrounded while still embryonic by a gelatinous connec- 

 tive tissue, its first accessory organ. Later on in development 

 this tissue becomes converted either to cartilage or bone, leav- 

 ing, however, a nearly uniform layer of the original tissue be- 

 tween it and the membrane. There is thus formed a mold 

 which reproduces the membranous labyrinth in its details, the 

 bony (or cartilaginous) labyrinth. The gelatinous tissue be- 

 comes soon converted into a serous fluid, called the perilymph, 

 in distinction from the endolymph of the interior, and the cav- 

 ities involved are conveniently distinguished as the perilym- 



