38 THE SKELETON OF THE C/IT. 



surface of the bone. The cranial end of the ridge is pierced 

 by an oval foramen through which the frontal sinus (Fig. 43, 

 m, in'), 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 

 lamellae of the ethmoid. 



The medial border forms a vertical plate {d), broadest 

 craniad and roughened for articulation with its fellow of the 

 opposite side except at its cranial end, where it articulates v/ith 

 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, n), 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 



