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 Cervide (deer) 
horns are usually developed only on the male, and are shed 
every year. In the Bovidz (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 
