48 PLEISTOCENE MAMMALIA. 



does not reach so high as the parietal. The lower internal surface of the thick rounded 

 head of the wedge is concave and smooth, and is closely applied to the posterior 

 surface of the tympanic bulla, with which it has no further connection. The free 

 end bears the articular surface for the stylohyal. This point, therefore, represents 

 the styloid process in man. We have ascertained that in this bone there are no 

 mastoidal cells, similar to those in the mastoid of man, connected with the tympanic, their 

 absence in the Eelidse being compensated for by the large tympanic bulla. In spite of 

 this significant fact, bearing on the "vertebral theory " of the skull of Carnivora, we use 

 Professor Owen's name of " mastoid" for the bone in question, without discussing the value 

 of the many and conflicting theories of the homologies of the component parts of the 

 cranium. 



Tpnpanic. — The tympanic bone consists of two portions — the tympanic proper, and 

 the bulla or supplementary portion. The tympanic proper forms a somewhat compressed 

 oval chamber, the outer, upper, and posterior walls of which are in part firmly articulated 

 to, and partly formed by, the inferior edge of the squamosal between the glenoid cavity 

 and the mastoid, and which is directed from the latter downwards, inwards, and slightly 

 forwards, parallel to the tympanic bulla. Externally it presents an oval opening under 

 the supra-mastoid ridge of the squamosal — the "meatus auditorius externus" (w). This 

 is the original portion of the bone which is in the young animal simply a thin plate, 

 resembling a horseshoe in form, attached by the two ends to the squamosal, which thus 

 completes the ring. Across it is stretched the membrana tympani, like the parchment 

 over the head of a drum, the centre receiving the handle of the malleus. The outer sur- 

 face of the' chamber curves slightly forwards, downwards, and inwards {n), in front of the 

 bulla, and it is much roughened for the attachment of the posterior branches of the 

 stylo-maxillary ligament. In the anterior part of the articulation with the squamosal 

 is the glenoid fissure. The anterior end of the bone turns downwards, and forms one or 

 two small, thin, hooked processes (;«), which are irregular in shape and number, and 

 overhang the foramen lacerum medium, a large irregular cavity formed by the junction of 

 the bulla, the posterior processes of the basi- and ali-sphenoids, and the petrosal. It 

 transmits the canals for the Eustachian tube, the groove for the Vidian nerve and artery, 

 and has within it the external orifice of the foramen caroticum. It is called, erroneously, 

 in our opinion, " le trou dechire anterieur " by Straus-Durckheim. 



Inside the "meatus auditorius" a long septum or curtain partially divides the 

 chamber, and forms a deep groove, open downwards, which passes under and in front of 

 the " meatus." The inner wall of the chamber is formed by a thin plate of bone, that 

 divides it from the bulla. It is said by Straus-Durckheim to be double in the cat. We 

 have, however, examined it in the adult cat with a powerful microscope, and it appears 

 to be homogeneous in structure throughout. The lower surface of the petrosal roofs in 

 and thus completes the chamber, being soldered to the tympanic by exceedingly thin 

 though firm connections at the posterior and upper borders of the cavity. The tympanic 



