384 THE SENSORIAL FUNCTIONS. 



to the membranous canals contained within 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 imitation 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 sac- 

 culus. 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 intermedium 

 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 labyrinth of all the mammalia, 

 and from their much greater size and hardness in 

 aquatic animals, there can be little doubt that they 

 perform some office of great importance in the phy- 

 siology of hearing. t Their size and appearance in 

 the Dog are 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 sud- 

 denly terminates in a curved point, or hook (h), 



* Scarpa and Weber term it the sinus, or alveus utricnlosus ; it 

 is called by others the sacculus vestibuli. Breschet gives it the name 

 of le sinus median, 



t These cretaceous bodies are termed by Breschet otolithes, and 

 otoconies, according as they are of a hard or soft consistence. 



