FRONTAL SINUSES. 3 



they may extend upwards as far as the frontal eminence, or fully two inches 

 above the naso-frontal suture, and outwards over the orbit into the base of 

 the external angular process ; or they may exist only as slight recesses in the 

 nasal portion of the bone. The dimensions of the sinuses are not necessarily 

 related to the degree of prominence of the glabella and superciliary ridges, 

 which -are sometimes strongly marked without being excavated by the air- 

 spaces ; while on the other hand large sinuses not unfrequently co-exist with a 

 comparatively flat lower frontal region, having apparently been formed by the 

 recession of the inner table of the bone. The right and left sinuses are separated 

 by a thin osseous partition, which is seldom defective ; but they are often unequally 

 developed, so that the septum deviates strongly from the median plane. In extreme 

 cases one sinus may extend equally, or nearly so, in both halves of the frontal bone, 

 the cavity of the opposite side being either rudimentary or wanting. The lower 

 part of the sinus tapers into the infundibulum, a narrow passage which leads down- 

 wards and backwards through the fore part of the lateral mass of the ethmoid into 

 the middle meatus of the nose. The infundibulum is deeply placed behind the 



right frontal sinus 



':-': ' 



' \ .. 



passage into right left frontal sinus 



nasal cavity. 



Fig. 2. LOWEK PORTION OF A FRONTAL BONE, SHOWING UNSYMMETRICAL DEVELOPMENT OF THE 



FRONTAL SINUSES. (From a photograph by G. W. B. Waters.) (GK D. T.) 



nasal process of the superior maxillary bone, and near the inner wall of the orbit 

 (</. Osteology, fig. 66) ; its termination in the middle meatus is about on a level 

 with the palpebral fissure. 



Vessels and nerves of the scalp. The supraorbital nerve and artery pass 

 almost vertically upwards from the supraorbital notch, and more internally the 

 frontal artery and supratrochlear nerve ascend over the margin of the orbit, while 

 the large frontal vein descends in a similar position to the root of the nose. 

 Posteriorly, the occipital vessels and great occipital nerve run upwards to the vertex, 

 entering the scalp somewhat internal to a point midway between the external 

 occipital protuberance and the mastoid process. The superficial temporal artery 

 crosses the base of the zygoma immediately in front of the ear, and its anterior 

 branch can frequently be seen, especially in old persons, running upwards and 

 f 01 wards with a tortuous course over the fore part of the temporal muscle towards 

 the forehead. 



Endocranial blood-vessels; In contact with the inner surface of the cranial 

 wall the superior longitudinal sinus is directed backwards along the middle line, 

 extending from the lower part of the forehead to the external occipital protuberance. 



B 2 



