212 MAMMALIAN ANATOMY 



(Fig. 137) is egg-shaped, smooth, and transparent. It is sometimes 

 marked by faint, oblique grooves which indicate thickenings or partial 

 septa on the internal surface. The anterior end is prolonged into a 

 sharp spine which joins the under surface of the sphenoid between 

 the basisphenoid and the alisphenoid. The internal edge is slightly 

 arcuate, and is impressed, behind the middle, by one or more faint 

 vertical grooves which may be deepened into canals. Behind these 

 grooves is a slight emargination for the anterior outer wall of the 

 jugular foramen. The posterior end is rounded and exhibits two 

 roughened places for attachment to the paroccipital process, of the 

 occipital and to the mastoid. Between the entotympanic and the 



P 



ectotympanic, behind, is a groove which forms the inner wall of the 

 stylo-mastoid foramen and the terminal portion of the extrapetrous 

 part of the aquseductus Fallopii. Just on the inner side of this 

 foramen is the deep pit for the tympano-hyal bone. The outer 

 boundary of the entotympanic is the line where the rougher and 

 harder ectotympanic has become fastened to it. The line is about 

 parallel with the long axis of the bulla, but curves upward in front 

 and behind. Its course is marked by numerous small foramina. 

 Between its anterior end and the spine on the front of the bulla is 

 a very short slightly emarginate anterior boundary which contributes 

 to the inner part of the lower margin of the Eustachian opening. 

 A minute spine near its inner part assists in forming a small notch. 



The shape of the external surface of the ectotympanic (Fig. 141) 

 is almost that of a ring. Its width is about the same in all parts, 

 except in front, where it is prolonged to an angle. It is narrowest 

 behind, where it appears to spring from the squamous as a round 

 column in front of the stylo-mastoid foramen. It has a distinct, 

 arcuate and slightly sinuate, anterior outer and upper margin, which 

 abuts against the squamous and is directed, from behind, forward and 

 then downward and inward. It forms the lower margin of the 

 Glaserian fissure ; near its middle is a notch for the canal of Huguier. 

 At its inner end, formino- nearlv a ri - ht ano;le with it, is a short 



O / O O 



anterior upper and inner margin which is directed inward and back- 

 ward to meet the front margin of the entotympanic. This part of 

 the border is emarginate and forms the lower part of the Eustachian 

 opening. The external surface of the ectotympanic faces outward and 

 somewhat downward, and in front also forward. It is rough and 



