158 THE SENSES. 



resemblance to a snail-shell. It is divided into two parts, by bony 

 and membranous structures, which run parallel from base to apex 

 of the spiral (Fig. 45). The upper passage opens into the vesti- 



FIG. 45. 



Bony Cochlea of the Human Ear, right side, opened from its anterior face (Cruveilhier). 



bule, and is known as the scala vestibuli ; the lower, the scala 

 tympani, is shut off by a membranous partition, which covers the 

 foramen rotundum. from the cavity of the tympanum. The scala 

 vestibuli is subdivided by a fibrous membrane (of Reissner) which 

 passes from the bony lamina spiralis to the wall of the scala ves- 

 tibuli. shutting off a triangular space (canalis cochlearis). The 

 floor of this space is the membranous partition (membrana basi- 

 laris) which separates the scala tympani from it, and upon this 

 membrane is the organ of Corti. 



What is the function of the cochlea ? 



It is to spread out, over as large a surface as may be, the mech- 

 anism for the reception of sounds by the organ of Corti, the end- 

 organ of the auditory nerve. It seems to be especially well adapted 

 for this purpose, because the solid spiral lamina connects it with the 

 bony framework of the skull, while it is contained in the mem- 

 branous labyrinth, whose fluid contents fit it for the response to 

 vibrations. It is further insulated by its suspension in the peri- 

 lymph. These peculiarities favor the conduction of semi-vibra- 

 tions through the bone, as well as of vibrations through the 

 mechanism of the middle ear. 



