240 ELEMENTS OF MAMMALIAN ANATOMY. 



cavity, the bony cochlea, extending cephalad from the 

 vestibule. If both chambers of the auditory bulla are 

 removed and a bristle thrust into the fenestra rotundum 

 (Fig. 17), it will enter the basal whorl of the bony cochlea. 

 A line drawn from the lateral margin of the foramen 

 ovale to the mesal margin of the fenestra rotundum 

 passes through the apex and middle of the base of the 

 cochlea, which may be rendered visible by carefully 

 clipping off with the bone-forceps the ventral portion 

 of the petrous bone along the line indicated. This 

 coiled canal, the cochlea, is divided into two channels 

 by a shelf of bone, the lamina spiralis, projecting from 

 the central axis or modiolus of the coil (Fig. 109). The 

 bony lamina extends but partly across the canal, the 

 remaining distance being bridged by membrane. The 

 cephalic channel, or the one nearer the apex of the 

 cochlea, is called the scala vestibuli. The other is 

 the scala tympani. 



The semicircular canals probably have nothing to do 

 with hearing, as they are well developed in fishes, w T hich 

 do not hear at all. They may aid in helping the cat to 

 maintain its equilibrium. The auditory nerve, how- 

 ever, is distributed to the vestibule and semicircular 

 canals as well as to the cochlea upon the lamina spiralis, 

 where the organ of Corti, the essential organ of hearing, 

 is located. 



REMARKS ON THE MAMMALIAN NERVOUS SYSTEM. 

 So far as known, the relation of the sympathetic to the 

 peripheral and central nervous systems is the same in 

 all mammals. The number of spinal nerves varies with 

 the number of vertebrae. The distribution of these 

 nerves, however, is approximately the same in all forms 



