HEARING. 305 



same description applies in all respects botli to the osseous 

 and to the membranous canals contained witliin them; the 

 space (p) which intervenes between the two, being filled with 

 the perilymph. But the form of the membranous vestibule 

 demands more particular notice, as it is not so exact an imi- 

 tation of that of the osseous cavity; being composed of two 

 distinct sacs, opening into each other: one of these (u) is 

 termed the utricle;'^ and the other (s,) the sacculus. Each 

 sac contains in its interior a small mass of white calcareous 

 matter, (o, o,) resembling powdered chalk, which seems to be 

 suspended in the fluid contained in the sacs by the interme- 

 dium of a number of nervous filaments proceeding from the 

 acoustic nerves (g and n,) as seen in Fig. 396. From the 

 universal presence of these cretaceous substances in the la- 

 byrinth of all the mammalia, and from their much greater 

 size and hardness in aquatic animals, there can he little 

 doubt that they perform some office of great importance in 

 the physiology of hearing.t Their size and appearance in 

 the Dog is shown in Fig. 397: and in the Hare, in Fig. 398. 

 The Cochlea, again, is an exceedingly curious structure, 

 being formed of the spiral convolutions of a double tube, or 

 rather of one tube, separated into two compartments by a 

 partition (l,) called the lamina spiralis, which extends its 

 whole length, except at the very apex of the cone, where it 

 suddenly terminates in a curved point, or hook (h,) leaving 

 an aperture by which the two portions of the tube commu- 

 nicate together. In Fig. 395, a bristle (b, b) is passed through 

 this aperture. The central pillar, round which these tubes 

 take two and a half circular turns, is termed the modiolus. 

 Its apex is seen at (m.) One of these passages is distin- 

 guished by the name of the vestibular tiibe,X in consequence 



• Scarpa and Weber term it the sinus or alveus titrlculosus ,- it is called by 

 others the sacculus vestihuU. Brcschct ^ives it tlie name of Ic sinus median. 

 See the Memoir already quoted, p. 98. 



I These cretaceous bodies are termed by IJrcschct otolithcs, and otoconicSy 

 according as they are of a hard or soft consistence. Ibid. p. 99. 



\ Sea la vestibuli. 



Vol. II. 39 



