THE LABYRINTH. 445 



micircular canals; one anteriorly leading into the upper scala 

 of the cochlea: and the last placed in its internal paries is the 

 aqueduct of the vestibule. In addition to these orifices, the pa- 

 rietes of this cavity are cribriform in the fossa semi-elliptica 

 and near the foramen rotundum.* 



The Semicircular Canals (Canales Semicirculares) are at the 

 posterior extremity of the vestibulum. They are three in num- 

 ber, and are named from their relative situation, Superior or 

 Anterior, Posterior or Inferior, and External. Each one forms 

 rather more than the half of a semicircle, and has its cavity 

 about half a line in diameter: their orifices are somewhat di- 

 lated beyond this measurement. The apparent thickness of 

 their parietes is greater in the adult than in the infant. 



The Superior Canal runs from without inwards and back- 

 wards. Its anterior orifice is above the fenestra ovalis, and is 

 enlarged into an ampulla or elliptical cavity. At its posterior 

 extremity, it joins the upper extremity of the inferior canal, so 

 that a common trunk is thus formed, the orifice of which is at 

 the internal posterior part of the vestibulum, and is dilated into 

 the shape of a funnel.t 



The Posterior or Inferior Canal is nearly vertical; has its 

 concavity in front, and its convexity behind, and joins, as just 

 remarked, with the superior ; its inferior orifice, which is near 

 the foramen rotundum, is also enlarged into an ampulla or ellip- 

 tical cavity. It is the longest of the three canals, and has its 

 ends nearer together. 



The External Canal is nearly horizontal, and is placed in the 

 space left by the divergence of the other two. It is the shortest 

 and the largest of the three. Its exterior orifice is also enlarged 

 into an ampulla or elliptical cavity, and is just behind the fora- 

 men ovale, or below the ampulla of the upper canal; the inter- 

 nal orifice is below the common opening of the other two ca- 

 nals. 



It is the union of the superior and of the posterior canals at 

 one of their extremities, which reduces the number of openings 



Ant. Scarpa, loc. cit. t Scarpa, loc. cit 



VOL. II. 39 



