ORGAN OF HEARING. 



531 



Of the vertical, one is anterior and superior, 

 the other posterior and inferior. The horizon- 

 tal is ejitemal. 



Superior vertical semicircular canal, canalis 

 semicircularis verticalis superior. The superior 

 vertical semicircular canal has its arch directed 

 upwards, and its extremities, which are more 

 widely divergent than those of either of the 

 other two semicircular canals, downwards. Fol- 

 lowed from its outer extremity, it describes its 

 curve from without and upwards, then down- 

 wards and inwards, with an inclination from 

 before backwards, in a word, across the petrous 

 bone. The convexity of the curve of this semi- 

 circular canal can always be recognized on the 

 upper surface of the petrous bone. The con- 

 cavity of it is free in the foetus and in the adult 

 of some of the lower animals, as the dog, hare, 

 &c. 



The inner extremity of the superior vertical 

 semicircular canal and the upper extremity of 

 the posterior vertical unite to form a common 

 canal, canalis communis, which is about one- 

 eighth of an inch long, and somewhat wider 

 than either of the two which unite to form it. 



Posterior vertical semicircular canal, canalis 

 semicircularis verticalis posterior. Leaving the 

 common canal, the posterior vertical semicircu- 

 lar canal describes its curve parallel to the 

 inner and posterior surface of the petrous bone, 

 perpendicularly from above backwards, then 

 downwards and forwards. The convexity of 

 the curve is thus directed backwards and 

 slightly outwards, its extremities forwards and 

 inwards. 



Horizontal semicircular canal, canalis semi- 

 circularis horizontalis. This is the shortest of 

 the three canals ; traced from its anterior extre- 

 mity, which is close to that of the superior ver- 

 tical, it curves outwards and backwards, then 

 inwards and forwards. Its convexity is out- 

 wards, its extremities directed inwards. 



We described in the vestibule three horns, 

 into the posterior and into the superior of 

 which the semicircular canals opened. In the 

 superior horn is observed the orifice of the ex- 

 ternal extremity of the superior vertical semi- 

 circular canal, and immediately below that and 

 above the fenestra vestibuli, the orifice of the 

 anterior extremity of the horizontal semicircular 

 canal. Both of these orifices are dilated into 

 ampullae. In the posterior horn is the orifice 

 of the canal common to the two vertical semi- 

 circular canals. Below and in front of this 

 orifice is the opening of the inferior extremity 

 of the posterior vertical semicircular canal. 

 Above the latter and immediately outside the 

 former is the opening of the posterior extremity 

 of the horizontal. Of all these orifices in the 

 posterior horn, that of the lower extremity of 

 the posterior vertical semicircular canal is the 

 only one which is dilated into aij ampulla. 



There are thus three ampullary dilatations, 

 one at the outer extremity of the superior verti- 

 cal semicircular canal, a second at the anterior 

 extremity of the horizontal, and the third at the 

 lower extremity of the posterior vertical. In 

 the lower and anterior wall of the ampullary 

 sinus of the posterior vertical semicircular canal 



is a small sieve-like spot indicating the entrance 

 of nervous filaments. 



The cochlea, (cochlea; Fr. le lima$on; Germ. 

 die Schnecke.) The cochlea does not exist in 

 all its perfection except in the Mammifera. In 

 birds it is in a very rudimentary state, but it is 

 easy to trace parts analogous to what we find 

 in the Mammifera. In regard to frequency of 

 occurrence in the animal series, the cochlea 

 does not stand next to the semicircular canals ; 

 the tympanum is found in a greater number of 

 animals. 



The cochlea forms the anterior part of the 

 labyrinth, and is, perhaps of all the parts of 

 the ear, that of which it is the most difficult to 

 give, either by descriptions or delineations, a 

 correct idea. If we can figure to ourselves a 

 tube tapering towards one extremity where it 

 ends in a cul-de-sac, and coiled, like the shell 

 of a snail, round an axis or central pillar ; and 

 if we suppose this tube subdivided into two 

 passages by a thin partition running throughout 

 its length, and of course spirally round the 

 axis, we shall have some conception of the 

 disposition of the cochlea. 



The tube of which the cochlea is composed, 

 canalis spiralis cochlea, is about an inch and a 

 half long, about one-tenth of an inch in dia- 

 meter at its commencement, and about one- 

 twentieth of an inch at its termination. It 

 describes two turns and a half, and that in a 

 direction from below upwards from left to 

 right in the right ear, and from right to left in the 

 left ear. The apex of the coil, which is also the 

 apex of the tube itself, is directed forwards and 

 outwards. The commencement of the first 

 turn of the cochlea forms an eminence towards 

 the cavity of the tympanum, called the pro- 

 montory. The second turn lies at its com- 

 mencement within the first, and only towards 

 its termination rises decidedly above the level 

 of it. By the base of the tube the cochlea is 

 connected with the vestibule. The cul-de-sac 

 at the apex forms a sort of vaulted roof called 

 cupola. 



The axis, or central pillar, modiolus s. colu- 

 mella cochlete. The first turn of the cochlea 

 takes a wider circular sweep than the rest, a 

 sweep having an average diameter of a quarter 

 of an inch, and is separated from the second 

 turn by the interposition of a soft bony sub- 

 stance, which extends also a little way between 

 the second and third. The axis, or central 

 pillar, as has been pointed out by Ilg,* is 

 nothing more than the internal walls of the 

 tube of the cochlea and the central space 

 circumscribed by their turns, in which space 

 the filaments of the cochlear nerve, running in 

 small bony canals, are contained. Now in con- 

 sequence of the wide sweep the first turn of 

 the cochlea takes in comparison with the rest, 

 the axis is very thick, about one-seventh of an 

 inch, where it is surrounded by the first turn, 

 and rapidly becomes thinner from the second 

 onwards to its termination. The last part of 

 it is in fact formed merely by the fold which 



* Einige anatomische 

 Prag. 1821, p. 7. 



Beobachtungen, etc. 

 2x2 



