THE NERVOUS SYSTEM. 227 
medial 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. 112). 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 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, some of which 
do not hear at all. They may aid in helping the cat to main- 
tain its equilibrium. The auditory nerve, however, is dis- 
tributed 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 vertebre. 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 digit or digits is want- 
ing. The arrangement of the columns or tracts of fibers in 
the spinal cord is very similar in allthe orders. The anterior 
or direct pyramidal tract, however, is partially or wholly 
absent in most orders below the Primates. It is best devel- 
