No. I.] THE VERTEBRATE EAR. 57 



almost always reach to the floor of the canal. The processes 

 concerned in this uplifting cannot be detailed here further than 

 to say that the bases of the rods of Corti spread apart while the 

 apices remain united, and owing to the plough-shaped foot of each 

 arch, they are readily forced under the neighboring cells. Work- 

 ing with this process, and perhaps more prominent than it, is 

 the process of growth in the basilar membrane, by means of 

 which the feet of the pillars are carried further apart. By the 

 resorption of the Sauropsid organ and the growth in height of 

 the whole organ of Corti, the latter comes to project from the 

 basilar membrane into the cochlear canals, a long, well-rounded 

 ridge, from the apex of which five to seven more or less com- 

 plete-^ rows of hairs project in a sweeping curve over towards 

 the foot of Reissner's membrane. As compared with the older 

 conceptions of the structure of Corti's organ and the relations of 

 its parts, the organ, as I have here briefly sketched it, is a 

 simple and compact structure. There are many of the impor- 

 tant histological intricacies yet to be made out concerning the 

 cell parts and their relations to the parent and neighboring cells 

 and to the nerves, but these need not detain us here. 



The essentials of the organ of Corti are then, in brief, a long, 

 fluid-filled canal, with a series of nerves piercing its floor to pass 

 up among the cells of a ridge, the apex of which is crowned by 

 several rows of long filamentous hairs which arise from the tops 

 of the sensory cells with which the nerves connect. 



The sensory cells are arranged in groups, but, as might be ex- 

 pected, these groups are by no means as regular as they would 

 be were they free to multiply and shape themselves, as is the 

 case of the sensory cell groups in the surface canals of fishes. 

 It should be noted that after being shut up in the cranial box 

 the canal organs were so hemmed in that there was no room to 

 spare for the usual amount of canal space to accommodate a 

 single sense organ, and the larger patches of sensory cells, such 

 as the maculae utriculi sacculi and the papilla lagenae, were 

 formed in a manner quite unlike the more typical cristae of 

 the ampullae. With the lengthening of the lagenar pocket there 

 was afforded opportunity for the growth of a series of closely 

 related canal organs ; the opportunity was seized ; the sense 

 organs grew in all directions, practically covering the floor of 



1 There are usually four quite complete rows with straggling cells beyond. 



