38 THE SKELETON OF THE CAT. 
surface of the bone. The cranial end of the ridge is pierced 
by an oval foramen through which the frontal sinus (Fig. 43, 
m, m'), which lies within the ridge, communicates with the 
spaces in the ethmoid bone (nasal cavity). Craniad of the 
ridge the surface (Fig. 26, c) is rough and, together with the 
raised medial border of the bone and the orbital plate, encloses 
a rectangular space which in the natural state receives a portion 
of the labyrinth of the ethmoid. ‘The ventral surface is marked 
at its medial edge by a thin longitudinal ridge which, when the 
bones are articulated, is continuous with one of the vertical 
lamellz of the ethmoid. 
The medial border forms a vertical plate (¢), broadest 
craniad and roughened for articulation with its fellow of the 
opposite side except at its cranial end, where it articulates with 
the border of the nasal bone. 
The caudal border is roughened, bevelled at the expense 
of the outer surface, and articulated with the parietal bone 
except at its ventral end, where it articulates with the alisphe- - 
noid. 
The lateral border is smooth, and it is here that the orbital 
plate is joined to the frontal plate at right angles. Along its 
cranial two-thirds this union is marked by a sharp ridge, the 
supraorbital margin (Fig. 40, 0) or arch. This ridge extends 
caudolaterad as a triangular projection, the zygomatic (or 
postorbital) process (Fig. 40, ), which is flattened on its 
cranioventral face near its extremity and forms part of the 
boundary of the orbital fossa. At its cranial end the lateral 
border articulates with the nasal and maxillary bones. 
‘The orbital plate (Fig. 40, 5’) arises from the ventral surface 
of the lateral border of the frontal plate. It is directed ventrad, 
is smooth and concave on its outer surface, and forms the dorsal 
portion of the medial wall of the orbital fossa. Near its ventral 
border it bears the small ethmoidal foramen, for the artery of 
the same name. 
On the caudal one-half of its inner surface (Fig. 26) it 
assists the caudal part of the dorsal plate in forming the brain- 
case. The cranial one-half of its inner surface is marked off 
from the remainder of the surface by a sharp irregular ridge 
