24 THE BONES AND JOINTS. 



a ridge runs obliquely outwards and downwards, in winch lies 

 the suture between this bone and the prootic bones. This bony 

 ridge (prod&gu* mastoicleus, autt.) is usually cartilaginous in R. escu- 

 lenta, even in old animals ; in R. temporaria, even in young speci- 

 mens, it is bony. In the latter species the bones unite very early, 

 while in the former they remain separated by the primitive carti- 

 lage. Between this crest and the processns condyloidcus there is a 

 depression (fossa condyloidea], with a hole (foramen condyloid< (a/i} 

 through which the vagus nerve leaves the cranium. The exoccipital 

 take part in the formation of the labyrinth of the ear, as will be 

 noticed later on. 



Articulation of the Exoccipital Bones with tlie Atlas. From the 

 middle of the anterior surface of the body of the atlas a ligament 

 arises, representing to a certain extent the lig. suspenwrium denfis, 

 and attached to the basal portions of the exoccipital bones. 



2. The prootic bones, ossa petrosa, Cuvier (Figs. 10, n, and 

 lip). 



Cuvier, roclier, I. c., p. 388, PI. XXIV, ee. Duges, n. 12, rupeo-ptereal. 

 Stannius, ala temporalis. Meckel, Schadelstiick des Schla- 

 fenbeins. Parker and Bettany, I.e., prootic. 



Thoe paired bones lie at the sides and in front of the exoccipital 

 bones. As already explained, they remain in R. escidenta separated 

 from these by cartilage, while in R. temporaria they early enter into 

 bony unflm with them ; this is due to the complete ossification of 

 the processus mastoidetts in the latter species, as stated above. The 

 prootics form the lateral expansions of the posterior part of the 

 skull in which the organs of hearing are placed. The large cavity 

 which contains the ear labyrinth is completed by the exoccipital : 

 internally it opens freely into the skull, and externally on the 

 posterior wall of the skull through the foramen ovale, which is 

 formed by both these bones. The postero-lateral part of the prootic 

 usually remains cartilaginous : at the side and in front of the 

 foramen ovale this cartilage is pierced by a small opening, through 

 which passes the nervns facialis or ramns tympanic tot n. ragi (Volk- 

 mann). At the side there is a process to which the suspensorium 

 of the lower jaw is attached (Fig. T 2 p] : behind this is a hollow in 

 which the auditory ossicles lie, and which may be designated fossa 

 tympanica (Fig. 12 (}. The anterior border of the bone forms the 



