262 THE EVOLUTION OF MAN. 



presented. In them, the sound-waves are conveyed directly 

 through the wall of the head itself to the internal ear. 



The inner apparatus, that which produces the sensation 

 of sound, receiving the sound-waves thus conveyed to it, 

 consists in Man, as in all other Vertebrates (with the single 

 exception of the Amphioxus), of a closed auditory sac filled 

 with fluid, and of an auditory nerve, the ends of which are 

 distributed over the wall of this sac. The vibrations of 

 the waves of sound are conveyed by that medium to these 

 nerve-ends. In the auditory fluid (endolymph), which 

 fills the labyrinth, and opposite the places at which the 

 auditory nerves enter, are some small stones, composed 

 of a mass of microscopic calcareous crystals (otoliths). The 

 organs of hearing of most Invertebrates have essentially 

 the same construction. In them, also, it usually consists of 

 a closed sac filled with fluid, containing otoliths, and having 

 the auditory nerve distributed over its wall. But while in 

 Invertebrates the auditory vesicle is usually of a very 

 simple spherical or oval form, in all Amphirhina, on the 

 contrary, that is, in all Vertebrates above the Fishes up to 

 Man, it is distinguished by a very characteristic and singular 

 form known as the auditory labyrinth. This thin membra- 

 nous labyrinth is enclosed in a bony envelope of the same 

 form, the osseous labyrinth (Fig. 245), which lies within the 

 petrous bone of the skull. The labyrinth in all Amphirhina 

 is divided into two sacs. The larger sac is called the 

 auditory pouch (utricvlus), and has three curved appendages, 

 called the semi-circular canals (c, d, e) ; the smaller sac is 

 called the auditory sac (sacculus), and is connected with a 

 peculiar appendage, which in Man and the higher Mammals 

 is distinguished by a spiral form, like the shell of a snail, and 



