42 ELEMENTS OF MAMMALIAN ANATOMY 



to which the zygomatic process is attached; the mastoid (Fig. 

 1 8), which is the part caudad of the squamosal and dorsal 

 to the bulla; the tympanic, which forms the auditory, or tym- 

 panic bulla; and the petrous (Figs. i8 and 19), which contains 

 the internal ear. The squamous portion overlaps the parietal 

 dorsally in a scale-like manner and is limited ventrally by a 

 clearly defined projecting ridge extending above the external 

 auditory meatus as the dorsal border of the zygoma. 



The zygomatic process extends craniad to join the zygomatic 

 process of the malar, the two together forming the zygomatic 

 arch, or zygoma, to which the masseter muscle is attached. 

 Ventral of the root of the zygomatic process is the mandibular 

 fossa for the articulation of the condyle of the mandible. 

 Immediately caudad of this cavity is the postmandibular proc- 

 ess. The mastoid portion of the bone is somewhat triangular 

 in shape, about two centimeters long, and lies caudad of the 

 external meatus. 



The tympanic portion appears on the base of the skull as 

 the tympanic bulla. Its cavity is divided into two unequal 

 chambers by a bony septum rising from the floor and reaching 

 almost to the roof. The cranial or true tympanic chamber, 

 sometimes called the middle ear, is the smaller, and has on 

 its lateral wall a horseshoe-shaped prominence known as the 

 tyfnpanic ring, to which the tympanic membrane of the drum 

 of the ear is attached. There are four apertures in the walls, 

 of the cranial tympanic chamber: The meatus acusticus externus 

 or the external acoustic canal; the auditory or Eustachian tube, 

 leading from the cranial dorsal angle of the chamber to 

 the pharynx; the fenestra vestibuli and the fenestra cochlece, 

 opening into the internal ear. The caudal or medial chamber 

 of the bulla is larger than the cranial, but has no special 

 features worthy of consideration. 



The petrous portion of the temporal bones is not visible on 

 the external surface of the skull, but. may be seen by looking 

 into the external acoustic meatus. It forms the medial wall 



