SENSITIVE SYSTEIVl. 401 



in the fluid surrounding them. It was once supposed that the 

 auditory nerve was spread over the periosteum by which the 

 labyrinth is hned ; but later researches, however, have shewn that it 

 is within the saccuh it expands, which do not even come in con- 

 tact with the periosteum, but are simply connected to it by a 

 pellucid, cellular, mucous-like matter. 



The semicircular canals are formed out of a peculiarly hard 

 brittle bone ; and their diameter is so small, as scarcely to ad- 

 mit of the introduction of the head of a common pin. 



Cochlea. 



This is the last division of the labyrinth, and by far the 

 most complex one. It receives its name from its resemblance to 

 the convolutions of the shell of the snail. Possessing a spiral 

 or pyramidal form, it has (by no means inaptly) been com- 

 pared to a spiral staircase, running round a column placed in 

 the centre. It is situated below the vestibule ; its base resting 

 against the meatus auditorius internus ; its apex extending to 

 the Eustachian tube. At its base it describes a large circle 

 which winds upward, gradually decreasing towards the apex, 

 forming altogether about two turns and a half. If we make a 

 vertical section of the cochlea, it presents the appearance of being 

 divided into three cavities or separate compartments: this, how- 

 ever, is not the case in its integral state, since the spiral turn- 

 ings of the tube run from one into the other. 



Lamina Spiralis — Scala, Cochlea. — There exists a structure in- 

 side the cochlea, giving peculiar complexity to it, denominated 

 the lamina spiralis. It consists in a partition or plate set edge- 

 ways, partly osseous and partly membranous, running through 

 the spiral tube of the cochlea, and dividing it into two separate 

 gyrations, called the scala: cochlea, which at the apex run into 

 each other and communicate. At the base, the external gy- 

 ration communicates, through the fenestra rotunda, with the 

 cavity of the tympanum ; the internal gyration ends in the 

 vestibule. 



Modiolus. — This is the central column or pillar around which 

 the scalae perform their gyrations. It consists of a soft spongy 

 structure, being pierced on every side like a colander, for the 

 transmission of nervous filaments to the lamina spiralis. 



Infundibulum. — Towards the apex of the cochlea the modiolus 

 opens, the aperture bearing some resemblance to a funnel, being 

 full and expanded upwards : this aperture is what is called the 

 infundihulum. 



Meatus Auditorius Internus. — This is a small and entirely bony 

 canal, piercing the petrous portion of the temporal bone, and 



3 F 



