948 



THE SENSORY APPARATUSES. 



1. The Vestibule (Fig. 515, /). 



This is a small, somewhat oval cavity, in the centre of the bone, and outside 

 the perforated bony plate that forms the bottom of the internal auditory hiatus. 

 It is a real vestibule, with regard to the other parts of the labyrinth, which all 

 open into it. 



On its external ivall is the fenestra oralis {fenestra vestibvU), an opening 

 closed by the stapes. The inn<^ wall shows the foramina through which the 

 filaments of the vestibular branch of the auditory nerve pass. Below, and in 

 front, is a large orifice, the commencement of the scala cochle* ; above, are five 

 little apertures, the openings of the semicircular canals. 



2. The Semicircular Canals (Fig. 513). 



Three in number, and very narrow, these canals owe their name to their 

 form. They are placed above the vestibule, like three semicircular arches 



Fig. 511. 



«tf 



DIAGRAM OF THE LABYRINTH IN FISHES, BIRDS, AND MAHMAI^ 

 I. FISH ; II. BIRD; III. MAMMAL. 



U, Utricle; S, saccule; US, utricle and saccule; Cr, canalis reuniens; R, recessus of the labyrinth; 

 VC, commencement of the cochlea ; C, cochlear canal ; L, logenulus ; K', cupola, forming the 

 summit of the cochlear canal ; V, caecum of the vestibule of the cochlear canal. 



united in a triangular manner at their base, and are distinguished as superior 

 or anterior, posterior, and external. The first two open together, by their adjacent 

 extremities, into the vestibule ; consequently, there are only five orifices of the 

 semicircular canals in this cavity. In addition, the adjoining openings of the 

 posterior and external canals are so close to each other, that they appear to be 

 sometimes united at the bottom of a short common canal. 



3. The Cochlea (Fig. 514). 



Situated behind, and below the vestibule, at the inner wall of the cavity of 

 the tympanum, the cochlea (snail-shell) is well named, as it presents exactly the 

 form of certain molluscs' shell. It is a spiral conical canal, twisting downwards, 

 forwards, and upwards, around a central conical axis (the modioUds, or columella) ; 



