SENSE OF HEARING. 66i 



the cochlea is divided longitudinally into two parts by a thin, bony par- 

 tition, the spiral lamina, which winds round its bony axis, following 

 its spiral turns, but limited externally by a free border. 



From the free border of the bony spiral lamina a fibrous membrane, 

 the membrana basilaris, extends outwardly quite to the external wall 

 of the cavity, to which it is attached. The common canal of the cochlea 

 is thus divided into two parallel passages or stairways, one above the 

 other. The superior of these passages communicates freely at its base 

 with the cavity of the vestibule, and is the scala vestibuli. The inferior 

 reaches to the fenestra rotunda, and is terminated by the membrane 

 stretched across this opening, which alone divides its cavity from that 

 of the tympanum ; it is accordingly known as the scala tympani. Both 

 these canals extend, in their spiral course, to the summit or cupola of 

 the cochlea. At this point a minute orifice of communication between 

 the two has been described by some writers, and doubted by others. 

 According to the observations of Buck, 1 it is probable that no such 

 opening exists in the natural condition of the parts, unless it be micro- 

 scopic in size. But whether the two canals communicate or not, at the 

 summit of the cochlea, the partition between them, throughout their 

 parallel course, is partly membranous ; and by this means an increase 

 or diminution of pressure upon the fluid of the vestibule at the fenestra 

 ovalis will be at once transmitted, through that of the scala vestibuli 

 and the scala tympani, to the membrane of the fenestra rotunda. Not- 

 withstanding, therefore, the incompressible character of the fluid of the 

 labyrinth, provision is made, to a certain extent, for the movement of 

 the stapes, according to the contraction or relaxation of the muscles of 

 the middle ear. 



But the septum above described, formed by the spiral lamina and the 

 membrana basilaris, is not the only longitudinal partition in the cavity 

 of the cochlea. The scala vestibuli is also divided into two parallel 

 canals, an internal and an external, by a thin membranous sheet which 

 starts from the upper surface of the spiral lamina near its outer border, 

 and extends upward and outward to reach the external wall of the coch- 

 lear cavity. As this membrane leaves the plane of the spiral lamina 

 and membrana basilaris at an angle of about 45 or 50 degrees, it shuts 

 off from the scala vestibuli a separate canal of prismatic form, having 

 for its floor the membrana basilaris, for its outer wall the wall of the 

 cochlea, and for its upper boundary the oblique membranous partition 

 between it and the scala vestibuli. This canal contains the auditory 

 epithelium cells and the termination of the fibres of the auditory nerve. 

 It is therefore the essential part of the cochlea, and is termed accord- 

 ingly the ductus cochlearis, 



The ductus cochlearis terminates at the summit of the cochlea by a 



1 On the Mechanism of Hearing, Prize Essay of the Alumni Association of the 

 College of Physicians and Surgeons, New York. Published in the New York 

 Medical Journal, March, 1874. 



