26 ELEMENTS OF MAMMALIAN ANATOMY. 



red marrow of the same nature as that in the cancellous 

 tissue of the long bones, wherein the red blood-corpuscles 

 are formed. The large medullary cavity or canal is filled 

 with yellow or fatty marrow. 



BONES OF THE SKULL. 



The skull is usually considered in two parts, the cranium 

 and face. The former is composed of nine bones, which 

 will be described in order. 



The frontal bones (Figs. 15 and 16) are two in number, 

 lying between the orbits, and articulating with one another 

 in the median line. They form the roof of the cranial part 

 of the brain cavity and the caudal portion of the roof of 

 the nasal chamber. A lateral projection (po, Fig. 16) is 

 known as the postorbital process. Within the frontal bone 

 is a cavity, the frontal sinus (Fig. 18), which contains air 

 and is lined with mucous membrane. It communicates with 

 the nasal cavity. The lateral descending portion of the 

 bone, articulating with the palatine and orbitosphenoid, is 

 the orbital plate of the frontal. 



In the majority of the Mammalia the frontal is a paired 

 bone, but in man the two portions become anchylosed dur- 

 ing the fifth or sixth year. The horns of ruminants are 

 outgrowths of these bones. Among the Cervidse (deer) 

 horns are usually developed only on the male, and are shed 

 every year. In the Bovidse (cattle) the horns are perma- 

 nent when present. 



The ethmoid (Fig. 18) is a single bone lying ventral to 

 the frontals and nasals. It separates the cranial cavity 

 from the nasal cavity and projects into the latter in the 

 form of two thin scroll-like plates of bone and a median 

 vertical plate. In order to see the relations of this bone, 

 three skulls must be used, one of which should be bisected 

 sagittally, a second should have the roof of the cranial and 



