24 THE BONES AND JOINTS. 



a riclg-e runs obliquely outwards and downwards, in which lies 

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

 ridg-e (processus, mastoideus, autt.) is usually cartilag-inous in R. escii- 

 lenta, even in old animals ; in B. 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 p-ocessns conchjloideiis there is a 

 depression [fossa condj/loidea), with a hole [foramen condf/Ioideum) 

 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 ILxoccipntal Bones with tlie Atlas. Prom the 

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

 arises, representing to a certain extent the lig. suspjensorhim dentis, 

 and attached to the basal portions of the exoccipital bones. 



2. The prootic bones, ossa petrosa, Cuvier (Figs. lo, ii, and 



Cuvier, rocher, I.e., p. 3S8, PI. XXIV, ee. — Dugès, n. 12, rupéo-ptéréal. 

 ^ — Stannius, ala temporalis. — Meckel, Schiidelstiick des Schlà- 

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



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

 bones. As already explained, they remain in B. esculenta separated 

 from these by cartilage, while in B. te^nporarla they early enter into 

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

 \hQ pirocessns mastoideus 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 extei'nally 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 nerviis facialis or ramus ti/mpanicus n. vagi (Volk- 

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

 of the lower jaw is attached (Fig. 127;) : behind this is a hollow in 

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

 tjjmpanica (Fig. 12 /). The anterior border of the bone forms the 



