844 THE OKGANS OF SENSE. 



ductus endolymphaticus and a small vein. The posterior part of the vestibule 

 receives the five rounded apertures of the semicircular canals ; its anterior part 

 leads, by an elliptical opening, into the scala vestibuli of the cochlea. This opening 

 is bounded inferiorly by a thin osseous plate, the lamina spiralis ossea, which 

 springs from the floor of the vestibule immediately lateral to the recessus 

 sphsericus, and forms, in the cochlea, the bony part of the septum between the scala 

 tympani below and the scala vestibuli above. From the anterior part of the floor 

 of the vestibule a narrow cleft, the fissura vestibuli, extends forwards into the bony 

 canal of the cochlea. It is bounded internally by the lamina spiralis ossea, and 

 externally by a second, smaller lamina, the lamina spiralis secundaria, which 

 projects from the outer wall of the cochlea. These two lamina are continuous 

 with each other round the posterior extremity of the fissura vestibuli. 



Canales Semicirculares Ossei. The osseous semicircular canals (Figs. 716, 71*7), 

 three in number, are situated above and behind the vestibule. They are dis- 

 tinguished from each other by their position, and are named superior, posterior, 

 and lateral. They open into the vestibule by five apertures, since the medial end 

 of the superior and the upper end of the posterior join to form a common canal 

 or crus commune. Differing slightly in length, each forms about two-thirds of 

 a circle, one extremity of which is dilated and termed the osseous ampulla. 

 They are somewhat compressed from side to side, and their diameter is from 1 to 

 1*5 mm., whilst that of the ampullae is about 2 mm. 



The superior semicircular canal, 15 to 20 mm. in length, is vertical and placed 

 transversely to the long axis of the petrous part of the temporal bone. Its 

 convexity is directed upwards, and its position is indicated on the anterior surface 

 of the petrous part of the temporal bone by the arcuate eminence. Its ampulla is 

 anterior and lateral, and opens into the vestibule immediately above that of the 

 lateral canal. Its opposite extremity joins the non-ampullated end of the posterior 

 canal to form the crus commune, which is about 4 mm. in length, and opens into 

 the upper and medial part of the vestibule. The posterior semicircular canal is 

 the longest and measures from 18 to 22 mm. Its ampulla is placed inferiorly, and 

 opens into the lower and back part of the vestibule, where may be seen about 

 six or eight small apertures (macula cribrosa inferior), for the transmission of the 

 nerves to this ampulla. Its upper extremity ends in the crus commune. The 

 lateral canal is the shortest ; it measures from 12 to 15 mm., and arches nearly 

 horizontally. Of its two extremities the lateral is ampullated, and opens into the 

 vestibule immediately above the fenestra vestibuli and in close relationship to 

 the ampullary end of the superior canal. 



Crum Brown (Journ. Anat. and Physiol, London, vol. viii.) pointed out that the lateral 

 canal of one ear is very nearly in the same plane as that of the other ; while the superior canal 

 of one ear is nearly parallel to the posterior canal of the other. 



Cochlea. 1 When freed from its surroundings the cochlea assumes the form of 

 a short cone (Fig. 720) ; the central part of its base corresponds with the bottom 

 of the internal acoustic meatus, whilst its apex or cupula is directed forwards 

 and laterally, and comes into close relation with the semicanal for the tensor 

 tympani muscle. It measures about 9 mm. across the base and about 5 mm. 

 from base to apex, and consists of a spirally arranged tube, which forms from 

 2 J to 2| coils around a central pillar, termed the modiolus. The length of the tube 

 is from 28 to 30 mm., and its diameter, near the base of the cochlea, 2 mm. Its 

 coils are distinguished by the terms basal, central, and apical ; the first, or basal 

 coil, gives rise to the promontory on the labyrinthic wall of the tympanic cavity. 



The modiolus is about 3 mm. in height, and diminishes rapidly in diameter from 

 base to apex, while its tapered extremity fails to reach the cupula by a distance of 

 1 mm. Its base corresponds with the area cochlesB on the fundus of the internal 

 acoustic meatus, and exhibits the tractus spiralis foraminosus, which transmits the 

 nerves for the basal and central coils of the cochlea and the foramen centrale, which 

 gives passage to the nerves for the apical coil. The foramina of the tractus spiralis 

 foraminosus traverse the modiolus, at first parallel to its long axis, but, after a 



1 In the following description the cochlea is supposed to be placed on its base. 



