236 ON THE STRUCTURE OF THE SKULL. 



downwards and forwards in the basi-sphenoid, opens into the 

 pituitary fossa, and lodges the internal carotid. 



The upper aperture of the anterior, narrow, passage from 

 the tympanum (/) is situated in front of the lip of the carotid 

 canal, and, at first, lies between the basi-sphenoid and the pro- 

 otic ; but, soon turning inwards, it enters the basi-sphenoid, 

 and passes beneath the carotid canal, to open into a much wider 

 median channel. The latter ends blindly in front and above, 

 behind and below the pituitary fossa ; but, inferiorly, it traverses 

 the substance of the basi-sphenoid, to open into the upper and 

 front part of the common Eustachian tube (Fig. 96, B). 



The posterior, wide, passage (b) leads downwards and inwards 

 through the substance of the basi-occipital, and the two passages 

 of the opposite tympana unite to form a short median canal, 

 which opens, on the front face of the basi-occipital, into the 

 common Eustachian tube (Fig. 96, A). 



The posterior tympanic passage has, however, another means 

 of communication with the exterior ; for, just before it joins 

 with its fellow, it gives off, downwards, a narrow canal, which 

 traverses the basi-occipital, and opens on its inferior face to the 

 outer side of, and a little behind, the aperture of the common 

 Eustachian tube (a, Fig. 96, A). There might, then, be said to 

 be three Eustachian tubes in the Crocodile, — two small and 

 lateral, one for each tympanum, and one large and median, 

 common to both tympana. 



Where the posterior tympanic passage passes into the tym- 

 panum, the ex-occipital presents a round aperture w T ith raised 

 edges, which is the anterior termination of the posterior division 

 of the canal for the internal carotid (C\ Fig. 96, B). In the in- 

 terval between this aperture of entrance and that of exit already 

 described, the internal carotid is unprotected by bone, and is 

 closely adherent to the outer surface of the cochlea ; which, held 

 by the cochlear hook already described, rests inferiorly upon the 

 fossa afforded by the basi-sphenoid and basi-occipital (CI). 



The posterior wall of the tympanum also exhibits, internally, 

 the aperture by which the eighth pair of nerves reaches the 

 exterior ; externally, those by which the portio dura leaves, and 

 the externa] carotid enters; superiorly, between the supra- 



