42 THE SKELETOX 



wliicli gives attachment to the structures which form collectively the membranous 

 cochlea. Tlie lamina makes two and a half turns in all. The first turn is the 

 largest, and forms a, liulging, the promontory, on the inner wall of the tympanum. 

 The lamina terminates at the aj)ex of the cochlea in a hooklike process — the 

 hamulus. 



Tlie modiolus is traversed by a central canal, and presents many canaliculi for 

 the transmission of the twigs of the cochlear division of the auditory nerve. There 

 is also a canal which winds round the modiolus at the ])ase. of the spiral lamina, 

 known as the spiral canal of the modiolus. 



The portion of tlie cochlea above the lamina is the scala vestibuli ; the part 

 below, that is, on the basal aspect of the lamina, is the scala tympani ; it opens 

 into the tympanum by way of the fenestra rotunda. Near the commencement of 

 th(> scala tympani, and close to the fenestra rotunda, is the cochlear orifice of the 

 aqueductus cochleae (ductus perilymphaticus). In the adult this opens on the 



Fig. 50. — The Cochlea in Saggital Section. (After Henle.) 



INTERNAL AUDITORY MEATUS 



THE SPIRAL CANAL 



inferior surface of the petrosal near the apex, and transmits a small vein from the 

 cochlea to the jugular fossa. 



Measurements of the principal parts connected with tlie auditory organs: — 



Internal auditory meatus .... Length of anterior Avail, 18-14 mm. 



" posterior wall, 6*7 mm. 



External auditory meatus . . . 14-16 mm. (Gruber. ) 



Tympanum Length, 13 mm. 



Height in centre of cavity, 15 mm. 

 Width opposite the membrana tympani, 2 mm. 

 " " tubal orifice, 3-4 mm. 



(Von Troltsch. ) 

 The capsule of the osseous labyrinth is in length 22 mm. (Schwalbe. ) 

 Superior semicircular canal measures along its convexitv 20 mm. 

 The posterior " " " " " " 22 mm. 



The external '' " " . " " 15 mm. 



The canal is in diameter 1*5 mm. (Huschke. ) 

 The ampulla of the canal, 2*5 mm. 



The Ossification of The Temporal Bone 



At ])iith the tcniporiil bone consists of three parts easily separable in the niaccrated skull : 

 they are the petrosal, sqiiamosfd, and the tympanic. (The styloid process is cartilaginous with 

 the exception of its basal element, the tynipano-hyal, which, with the ear-bones, will be described 

 with the appendicular elements of the cranium.) 



The sijuaniosal and tympanic bones develop in membrane. The squamosal is formed from 

 one centre, which appears as early as the eiahth week. Ossification extends into the zygoma, 

 which grows concurrently with the squamosal. At first the tymjianlc border is nearly straight, 

 but soon assumes its characteristic horseshoe; shape. At birth the post-glenoid tubercle is 

 conspicuous, and at the hinder end of the squamosal there is a recess where it comes into relation 

 with the mastoid antrum. The centre appears for the tympanic bone about the twelfth week. 

 At birth it is a horseshoe-shaped ossicle slightly aidcylosed to the lower border of the squamosal, 

 the open arms being directed upwards. The tip of the anterior arm terminates in a small 



