5 6 AYERS. [Vol. VI. 



to the present time. Relative to the mass of the body, the 

 longest cochlear tubes are found among the Chiroptera and the 

 Rodentia, while the Cetacea possess the relatively smallest. 



In the Mammalia the cochlear tube has become much more 

 completely separated from the sacculus than in any other group 

 of vertebrates. It is connected with the parent chamber by the 

 narrow sacculo-cochlear canal, — the canalis reuniens of Hensen. 



The term organ of Corti, as it is used in anatomy, is, strictly 

 speaking, not sufficiently exact for scientific purposes, and some 

 of the parts included in it have no functional relation to the 

 physiologically active parts of the mammalian cochlear organ. 

 Such parts as the auditory denticles and related structures have 

 certainly no intimate relation to the working of the organ to- 

 day, nor are they concerned in perfecting the organ structurally. 

 They are simply the remnants of a previous condition of struc- 

 ture. 



The mutual relations of the structures forming the organ of 

 Corti vary in different periods of the growth and life of the 

 organ. In the embryonic condition all the cells are firmly 

 planted on the floor of the canal, and this floor is attached on 

 both the inside and the outside edges to a thick cartilaginous 

 plate or bar, the derivative of the parallel cartilaginous bars 

 of the ancestral Sauropsids. 



As all the cells are in contact with this basement or basilar 

 membrane, which, although a thick plate at this time, thins out 

 later to form a tensely drawn membranous structure, and as 

 likewise all the cells take part in the formation of the free sur- 

 face of the organ, they all have the same length, both support- 

 ing and sensory cells : indeed, it is extremely difficult at this 

 early time to distinguish the supporting cells as such. 



The first change of importance is marked by the appearance 

 of the rods of Corti, by means of which the uniformity of the 

 cell arrangement is broken up. The rods as they develop 

 cause an elevation or upheaval of the two parts of the sensory 

 plate, between which they have arisen, so that from this time 

 on, in ever-increasing degree, till adult life is reached, the edges 

 of the organ lying on the uplifting limbs of the arches are raised 

 from the basilar membrane. The inner edge is disturbed the 

 most, and the inner row of hair cells and some of the supporting 

 cells no longer reach the basilar plate. The outer hair cells 



