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 rierve. In 
Ormthorhynchus (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 zygomatic 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 
eygoma., 
The zygomatic process extends craniad to join the zygo- 
matic 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 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 car, is the smaller, 
