Glass 6. Mammalia. 475 



(hard palate), whicli is formed of horizontal medianly apposed 

 portions of the premaxillae, maxillae and palatines* Behind, in the 

 septum between the cranium and the nasal cavities, which -was originally 

 cartilaginous, there is a bone with numerous fine perforations for the 

 olfactory nerves, the cribriform plate. From the front side of 

 this thin, folded, bony lamella arise, covered by a thin membrane 

 and projecting far into the nasal cavity ; they are known as the ethmo- 

 turbinal. Further forward on the outer wall, there is a bone composed 

 of a varying number of delicate bone lamellae, the maxillo- 

 turbinal, so that the greater part of the cavity is filled up. 

 There are larger or smaller air spaces (Fig. 386) in certain bones of the 

 head in connection with the nasal cavities of the Mammalia; especially 

 in the maxilla (maxillary sinus) and in the frontal (frontal 

 sinus); sometimes (in the Ox, Elephant, etc.) these sinuses are 



Fig. 386. Skull of an old Pig, sawn through longitudinally, in order to show the large 

 a r sinuses, h cranial cavity, I V I" air sinuses partially (I' the frontal sinus) divided 

 lay bony plates, s bony nasal septum. — After Bendz. 



of considerable size, extend into other bones, and are divided by 

 incomplete septa into a number of small cavities. Amongst other 

 characteristics of the skull it may be mentioned here, that a bony 

 bridge, the zygomatic arch,t runs from the articulation of the mandible 

 to the maxillge; it is formed by a process of the squamous (see below), 

 of the jugal, and sometimes a process of the maxilla (c/., the similar 

 bony bridge in Reptiles and Birds which is formed of the quadrato- 

 jugal and jugal). The hyoid consists of an unpaired body and 

 two horns on each side. The anterior horn, which corresponds 

 to the hyoid of Pish, is usually the longer, and consists of three 

 movable joints ; it is attached by its upper end to the skull (prootic) . 



* The palate is perforated in front at the junction of premaxillae and maxillae by 

 two openings {canales indsivi), through, which the Stensen's ducts, mentioned on 

 p. 333, pass. 



f In some Mammals there is a process near the middle of the jugal which meets 

 & similar one from the frontal and forms with this a bony bridge behind the eye. 



