THE SKELETON. 29 



lates with the vomer and presphenoid. The cribriform 

 plate is the caudal portion of the ethmoid (Fig. 18), which,, 

 extending transversely between the frontals, separates the 

 cranial cavity from the nasal cavity. It is pierced by many 

 pinhole foramina for the exit of the olfactory nerve. In 

 Ornithorhynchus (duck-bill of Australia) there is a single 

 large foramen in the cribriform plate, as is also the case in 

 birds. 



The temporal is a paired bone lying at the base and side 

 of the skull. It contains the organs of hearing. It con- 

 sists of four parts : the squamous or expanded portion (Fig. 

 16), to which .the sygomatic process is attached; the mastoid 

 (Fig. 17), which is the part caudad of the squamosal and 

 dorsal to the bulla; the tympanic, which forms the auditory 

 bulla; and the petrous (Figs. 17 and 18), 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 sygomatic process extends craniad to join the zygo- 

 matic process of the malar, the two together forming the 

 sygomatic arch, or zygoma, to which the masseter muscle is 

 attached. Ventral of the root of the zygomatic process is 

 the glenoid cavity for the articulation of the condyle of 

 the mandible. Immediately caudad of this cavity is the 

 postglenoid process. 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 auditory bulla. Its cavity is divided into two unequal 

 chambers by a bony septum rising from the floor and reach : 

 ing almost to the roof. The cranial or true tympanic 

 chamber, sometimes called the middle ear, is the smaller, 



