76 



INNERVATION. 



[CHAP. xvin. 



free, and directed towards the lamella, so as to leave an opening or 

 deficiency, the helicotrema of Breschet, by which the scalse tym- 

 pani and vestibuli communicate. The membranous zone connects 

 the convex border of the hook to the outer wall, and is also con- 



Fig. 136. 





Cochlea of a new-born infant, opened on the side towards the apex of the petrous bone. It 

 shows the general arrangement of the two scalae, the lamina spiralis, and the distribution of the 

 cochlear nerve. At the apex is seen the modiolus expanding into the cupola, where the spiral 

 canal terminates in a cul de sac. The helicotrema is not visible in this view. From Arnold. 



tinned upwards beyond the point of the hook, presenting, however, 

 towards the infundibulum, like the hook itself, a free concave bor- 

 der, contributing to form the orifice of communication. 



Such being the form of the osseous labyrinth, we may now pro- 

 ceed to consider the more delicate parts of the organ, and the im- 

 mediate distribution of the auditory nerves. We must premise that 

 the cavity of the osseous labyrinth is occupied by a limpid fluid 

 the perilymphj so called by De Blainville, from its surrounding, 

 though in the vestibule and semicircular canals only, a hollow 

 membranous apparatus, the membranous labyrinth, which latter 

 itself contains a similar fluid, the endolymph. 



Of the Structure of the Spiral Lamina of the Cochlea. We shall 

 term the two surfaces of this lamina tympanic and vestibular, as they 

 regard respectively the tympanic or vestibular scala. The osseous 

 portion of the spiral lamina extends more than half way from the 

 modiolus towards the outer wall, and is perforated, as already de- 

 scribed, by a series of plexiform canals for the transmission of the 

 cochlear nerves ; these canals, taken as a whole, lie close to the 

 lower or tympanic surface, and open at or near the margin of this 

 zone. The vestibular surface of the osseous zone presents in about 

 the outer fifth of its extent, a remarkable covering, more resembling 

 the texture of cartilage than anything else, but having a peculiar 

 arrangement quite unlike any other with which we are acquainted. 



