THE SKULL THE CEANIUM 225 



leave free a narrow space along the margin. The inner side of the 

 base is convex transversely, and is applied to the membrane in the 

 fenestra ovalis. 



The stapes forms with the incus an acute angle, sloping from the 

 head inward, upward, and backward. The long axis of the base is 

 about parallel with the zygomatic arch of the skull. 



THE BONY LABYRINTH. 



The petrous portion of the temporal bone conceals in its interior 

 the internal ear or membranous labyrinth, a system of small sacs and 

 tubes filled with fluid and bearing on their delicate walls groups of 

 sensory organs richly supplied with branches of the auditory nerve. 

 The cavities and canals of the petrous, which contain the internal ear, 

 constitute the bony labyrinth ; they are moulded so closely upon the 

 membranous labyrinth that they conform closely to it in size and in 

 shape. In young skulls the bony walls of the labyrinth are distinct 

 from the cancellous tissue of the petrous, but as this becomes con- 

 verted into compact tissue the outline gradually is lost. 



To understand the structure of the labyrinth it is necessary to 

 study it in a number of specimens and by three methods : (1) by 

 making casts of the interior in fusible metal, 1 thus converting the 

 cavities into solid bodies in order to show their shape and relative size 

 and position ; (2) by breaking away certain parts of the petrous and 

 looking into the cavities in order to examine the features of their walls 

 and the relation of their parts to the exterior of the bone ; and (3) by 

 cutting sections in various directions in order to establish the position 

 of the parts with reference to the planes of the petrous and of the 

 entire skull. 



The metal cast (Fig. 160) shows that the bony labyrinth comprises 

 (1) a posterior part, consisting of the semicircular canals; (2) a small 

 middle cavity, the vestibule ; and (3) a large anterior spiral canal, the 

 cochlea. 



The semicircular canals are three fine passages which traverse the 

 posterior part of the petrous in horseshoe curves and open through 

 their ends into the vestibule. From their general direction in the 

 skull they are known as the superior, the posterior, and the external 

 semicircular canal. Each canal lies, approximately, in one plane, and 



1 Bismuth, five parts ; lead, three parts ; tin, two parts. 



15 



