266 THE EVOLUTION OF MAN. 



various stages in its ontogenetic development still exist 

 permanently side by side in the ranks of the lower Vert.e- 

 brates.^'^ Even in Monotremes the snail-like spiral curving 

 of the cochlea is not present ; it is exclusively characteristic 

 of the other Mammals and Man. 



The auditory nerve (nervus acusticus), or the eighth 

 brain-nerve, — one of the main branches of which distributes 

 itself over the " snail " (cochlea), the other over the other 

 parts of the labyrinth, — is, as Gegenbaur has shown, the 

 sensory* dorsal branch of a spinal brain-nerve, the motor 

 ventral branch of which is the motor nerve of the facia] 

 muscles {nervus facialis). Phylogenetically it has, there- 

 fore, originated from an ordinary skin-nerve, and is, conse- 

 quently, of wholly different origin from the optic and 

 olfactory nerves, which represent the two direct processes 

 of the brain. In this respect the organ of hearing differs 

 essentially from the organs of sight and of smell. The 

 auditory nerve originates from the cells of the head-plate ; 

 therefore, from the skin-fibrous layer. From this also 

 develop all the membranous, cartilaginous, and bony cover- 

 ings of the ear-labyrinth. 



The development of the apparatus for the conveyance of 

 sound, situated in the middle and external ear of Mammals, 

 is entirely distinct from that of the apparatus of auditory 

 sensation. It must be regarded, phylogenetically as well as 

 ontogenetically, as an independent, secotidary formation, 

 which only afterwards connects itself with tlie primary 

 internal ear. Its development is, however, not less in- 

 teresting, and is equally clearly explained by Comparative 

 Anatomy. In all Fishes, and in the yet lower Vertebrates, 

 there is no special apparatus for the conveyance of sound. 



