44 ELEMENTS OF MAMMALIAN ANATOMY. 



ethmoturbinals. In most orders of animals five scrolls are 

 present, but in Echidna there are six and in some Ungu- 

 lates there are eight, while in adult Primates there are 

 only from one to three, more, however, being present 

 in the embryo. 



The mesethmoid is the perpendicular plate of bone 

 which, prolonged cephalad by cartilage, separates the 

 nasal cavity into two portions. Caudad it is united to 

 the cribriform plate, dorsally it articulates with the 

 median descending plates of the frontals and the nasals, 

 and ventrally it articulates with the vomer and pre- 

 sphenoid. 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 Ornithorynchus 

 (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 consists 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 1 8), 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 cephalad 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 



