252 ELEMENTS OF MAMMALIAN ANATOMY 



fenestra vestibuli. The superior semicircular canal lies in a 

 transverse plane cautlad to the preceding. The posterior semi- 

 circular canal lies in a vertical longitudinal plane, immediately 

 laterad from the jugular foramen. 



The cochlea is a coiled canal lying within the coiled cavity, 

 the bony cochlea, extending craniad from the vestibule. If 

 both chambers of the tympanic bulla are removed and a bristle 

 thrust into the fenestra cochleae (Fig. 18), it will enter the basal 

 whorl of the bony cochlea. A line drawn from the lateral 

 margin of the foramen vestibuli to the medial margin of the 

 fenestra cochleae passes through the apex and middle of the 

 base of the cochlea, which may be rendered visible by care- 

 fully 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. 121). The bony lamina extends but partly across 

 the canal, the remaining distance being bridged by membrane. 

 The cranial channel, or the one nearer the apex of the cochlea, 

 is called the seal a vestibuli. The other is the seal a tympani. 



The semicircular canals probably have nothing to do with 

 hearing, as they are well developed in fishes, some of which do 

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

 its equilibrium. The auditory nerve, however, 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 pe- 

 ripheral 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 with five digits. In those having a less 

 number of digits the nerve branch corresponding to the lacking 



