282 MAMMALIAN ANATOMY 



The external frontal sinus (Figs. 204, 205) is about as wide as 

 high, and deeper than wide. It forms the upper chamber of the nose. 

 It is open in front and below, but when the bone is in place the an- 

 terior opening is filled by the nasal and maxillary bones. Its inner 

 wall is the outer surface of the median plate, and the outer wall 

 the upper part of the inner surface of the orbital plate. The roof 

 (Fig. 205) is formed by the horizontal plate, prolonged in front into 

 the nasal process. On the roof is a longitudinal ridge which is con- 

 tinued down the short posterior wall of the cavity. This wall is a 

 partition which divides the external sinus from the internal sinus, and, 

 in the outer of the two sections formed by the longitudinal ridge, is 

 pierced by a foramen which permits more or less of the tip of the 

 upper external scroll (!') of the lateral ethmoid to protrude into the 

 internal sinus. The inner section on the roof receives the inner 

 uppermost scroll (1") of the ethmoid. 



The frontal sinus proper (Figs. 206, 207) is a cavity contained in 

 the upper part of the bone. It communicates with the nasal chamber 

 by the opening just described. Its floor is formed by the orbital plate 

 and the part of the horizontal plate over the cerebral surface which 

 may be called the cerebral plate. Its inner wall is formed by the 

 median plate, and its outer wall by the inner surface of the orbital and 

 temporal plates. Its posterior wall is the cerebral plate, and its roof is 

 the horizontal plate. It is often prolonged outward and backward into 

 the base of the postorbital process and downward into the constricted 

 part of the bone which separates the olfactory and anterior cerebral 

 fossae. Its walls are sometimes marked by ridges. 



The cerebral surface (Figs. 199, 203, 206) occupies the greater part 

 of the inner surface of the vertical plate of the bone. It consists of a 

 narrow, vertical, anterior portion and a large, cup-shaped, posterior por- 

 tion. The anterior portion faces inward, and is bounded in front by 

 the cribriform line, below by a short, straight, serrated, inferior border, 

 articulating with the orbitosphenoid. It rounds outward behind into 

 the second portion, and is arched above by a small surface which is con- 

 cave from side to side and joins the median plate. It forms the outer 

 and upper walls of the olfactory fossa for the olfactory lobe of the brain. 

 The larger cerebral portion is deeply concave and faces principally 

 backward and inward, forming, with the other frontal above, with the 

 cribriform plate of the ethmoid in front, and with the anterior sphenoid 



