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 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 digit or digits is want- 

 ing. The arrangement of the columns or tracts of fibers in 

 the spinal cord is very similar in all the orders. The anterior 

 or direct pyramidal tract, however, is partially or wholly 

 absent in most orders below the Primates. It is best devel- 



