MINUTE STRUCTURE OF THE COCHLEA. 761 



the outer part of the spiral canal, lying in the same plane as the osseous 

 lamina, and attached peripherally through the medium of a thick structure, 

 the spiral ligament. It increases in breadth from the base to the apex of 

 the cochlea, while the osseous spiral lamina diminishes in breadth. Thus 

 in the first turn of the cochlea this membrane forms about half of the 

 breadth of the septum made by it and the osseous lamina ; but towards the 

 apex of the cochlea the proportion between the two parts is gradually 

 reversed, until, near the helicotrema, the membranous part is left almost 

 unsupported by any plate of bone. 



Fig. 518. SECTION THROUGH ONE Fig. 513. 



OP THE COILS OP THE COCHLEA 



(altered from Henle). ^ 



A, the section is made in a 

 specimen softened by immersion in 

 hydrochloric acid ; S T, scala tym- 

 pani ; S V, scala vestibuli ; C C, 

 canalis cochleae ; R, membrane of 

 Reissner forming its vestibular /J t 

 wall ; I so, lamina spiralis ossea ; 

 1 1 s to I s p, lamina spiralis mem- 

 branacea ; I Is, limbus laminae 

 spiralis ; s s, sulcus spiralis ; n c, 

 twigs of cochlear nerve ; g s, gang- 

 lion spirale ; t, membrana tectoria; 

 6, membrana basilaris ; Co, organ 

 of Corti ; Isp, ligameutum spirale. 



The limbus lamincz spiralis (denticulate lamina of Todd and Bowman) 

 is a thick periosteal development near the edge of the osseous spiral lamina 

 on the side which looks towards the vestibular scala. It makes a some- 

 what convex elevation, presenting externally a sharp margin which over- 

 hangs that to which the basilar membrane is attached, being separated 

 from it by a groove. The groove is termed sulcus spiralis, and the margins 

 labium vestibulare and labium tympanicum respectively. The membrane of 

 Reissner (membrana vestibularis) arises from the inner part of the limbus, 

 and extends outwards at a considerable angle with the osseous spiral 

 lamina. 



The membrana tectoria (Claudius), or membrane of Corti (Kolliker), has 

 been variously described, but, according to the most recent researches, is 

 an elastic membrane attached on its one border close to the membrane of 

 .Reissner, and on the other by an extremely delicate portion to the peri- 

 pheral wall of the cochlea, a little above the membrana basilaris (Claudius 

 and Henle). It thus divides the canalis membrauacea into two parts : the 

 large part placed between it and the membrane of Reissner, and con- 

 taining endolymph ; the other, a narrow interval dividing it from the 

 membrana basilaris, and occupied by various cellular and rod-like structures 

 of a highly complicated description, which together are designated as the 

 organ of Corti. 



The canalis membranacea, or ductus cochlearis, bounded in the manner 

 already described, presents a blind pointed extremity at the apex and 

 another at the base. That at the apex extends beyond the hamulus, fixed 

 to the wall of the cupola, and partly bounding the helicotrema ; that at 

 the base fits into the angle at the commencement of the osseous spiral 

 lamina in front of the floor of the vestibule. Near to this blind extremity 

 the canalis membranacea receives a small duct, canalis reuniens (Hensen), 



