216 MAMMALIAN ANATOMY 



of the ridge on the outer edge of the basisphenoid. The point wherein 

 it ends enters the large notch on the outer part of the posterior border 

 of the basisphenoid. The outer surface of the ridge is fastened to the 

 outer bevelled edge of the same notch. Anterior and external to the 

 ridge the anterior border of the superior surface of the entotympanic 

 is crossed obliquely by a groove for the small internal carotid artery, 

 which enters the cranial cavity through a rudimentary middle lacerated 

 foramen formed between the sphenoidal notch, the entotympanic, and 

 the apex of the petrous. The sharp process at the anterior angle of 

 the entotympanic part of the superior surface is also fastened to the 

 basisphenoid in front of the notch. The upper surface of the ridge is 

 applied to the petrous, as is all the rest of the entotympanic portion. 

 At about the middle of the entotympanic part of the superior surface 

 are several transverse grooves, sometimes deepened into canals, for the 

 transmission of nerves and vessels from the base of the cranium. 



The ectotympanic part of this surface is much smaller and is not 

 continuous, inasmuch as the posterior third is lacking, except a small 

 round area which is attached to the posterior part of the under surface 

 of the squamous of the temporal. The gap thus left is filled by the 

 part of the rim of the auditory meatus on the squamous. The anterior 

 two-thirds are crescentic, the concavity of the crescent pointing inward 

 and backward. It is narrowest on the inside and widest near the 

 middle. It is divided into two parts by an alteration in the direction 

 of its plane which takes place along a middle longitudinal line. The 

 inner half is flat, and its plane faces slightly forward as well as down- 

 ward, nearly parallel with that of the entotympanic surface, but it lies 

 somewhat higher. This part is seen to be the groove which forms the 

 floor of the Eustachian tube, and which has been described as sepa- 

 rating the ectotympanic from the entotympanic portion. It is, in fact, 

 the upper inclined edge of the septum of the bulla. The part of the 

 ectotympanic surface lying outside of this groove is kidney-shaped ; 

 the long axis is directed obliquely inward and forward. Its inner, 

 sharper edge is emargiuate ; near its posterior termination is a hook, 

 which is directed forward, and behind which is a notch leading to 

 a groove for the chorda tympani nerve, crossing the surface obliquely 

 but almost parallel to the long axis of the skull. The rounded pos- 

 terior end is turned sharply upward and inward, and presents a smaller, 

 rough, triangular surface, facing upward, backward, and outward, which 



