THE SINUSES OF THE SKULL 97 



outwards. The upper portion of the anterior turbinated bone forms 

 the remainder of the inner boundary of the sinus, and separates it 

 from the cavity of the nose. 



The lower limit of this sinus extends to a point which may be 

 indicated by taking a line from the zygomatic ridge horizontally 

 forwards, about three and a half to four inches in length, the anterior 

 limit of which is about three inches from the nasal canthus. 



The frontal sinus is bounded by the frontal, nasal, lachrymal, 

 ethmoid, and anterior turbinated bones, and a point of considerable 

 importance to remember is that it is in direct communication with 

 the superior maxillary sinus of its own side. 



The outline of the sinus may be traced on the exterior of the 

 skull as follows : 



Its superior limit would be indicated by a transverse line drawn 

 from the middle line at a distance of half an inch above the supra- 

 orbital process, to the level of which it extends in the outward direction. 



The inner boundary in its upper portion is indicated by a straight 

 line representing the position of the septum between the two frontal 

 sinuses already referred to, and in its inferior portion by an oblique line 

 drawn downwards and outwards from the lower extremity of the above, 

 and which terminates on meeting the line, to be mentioned hereafter, 

 indicating the course of the lachrymal duct. 



The outer boundary of the sinus is very irregular. It takes at 

 first a sinuous course downwards from the line indicating its upper 

 limit, to the nasal canthus, and then takes a course obliquely down- 

 wards and forwards from this point in the direction of the nasal 

 peak. The line indicating the latter portion of its course will also 

 indicate the course of the osseous tube which accommodates the 

 lachrymal duct, and where this line intersects the oblique portion 

 of the line indicating the inner boundary of the sinus, we have its 

 lowest limit. 



The Inferior Maxillary Sinus. — The inferior maxillary sinus, like the 



