460 HENRY C. TRACY 



The ventral surface of the pterotic is triangular and exhibits a 

 small segment of the posterior osseous capsule (fig. 5, POC) 

 lateral to which there is a shallow groove (deepened by cartilage 

 in the fresh skull) which forms the posterior half of the hyoman- 

 dibular articulation. 



The greater part of the medial surface of the bone is deeply 

 beveled so as to face nearly medially, and roughened for the 

 attachment of the lateral cartilage plate (figs. 4 and 10). The 

 bone articulates posteriorly with the upright posterior part of the 

 exoccipital. A small vertical surface of the bone lies anterior 

 to this articulation and makes an open rounded angle with the 

 confluent surface of the exoccipital; through this angle the 

 superior sinus of the membranous labyrinth passes vertically. 

 A deep recess below extends back between the two parts of the 

 exoccipital and lodges the inferior end of the posterior semi- 

 circular canal and its ampulla. This recess is separated medially 

 from the saccular cavity by the canal for the vagus nerve. In 

 front of the vertical medial surface is a cone-shaped recess which 

 contains the ampulla of the lateral semicircular canal. 



Anteriorly, the body of the bone juts out over its base to meet 

 the backward projecting laminae of the falciform process of the 

 prootic. To the angle between these laminae, the pterotic pre- 

 sents an obliquely excavated surface and thus forms a chamber 

 for the reception of the lower end of the anterior semicircular 

 canal and its ampulla. The floor and outer wall of this chamber 

 is formed by the apposition of a thin curved plate of the pterotic 

 bone to the inferior lamina of the falciform process. Superiorly, 

 as has been mentioned above, the edge of the falciform process 

 does not quite meet the pterotic: the irregular slit thus formed 

 (figs. 3, 4 and 6) is closed by the lateral mass of cartilage which 

 is penetrated by the anterior semicircular canal. Below the 

 chamber the free edge of the projecting part of the pterotic is 

 continuous with the edge of the inferior lamina of the falciform 

 process and forms with it a low arch under which the utricular 

 part of the auditory recess communicates laterally with the 

 lateral recess. 



