330 Jacob Parsons Schaeffer. 
reduced to a papery delicacy. The base or median wall is directed 
towards the cavum nasi, and the apex of the sinus extends into 
the root of the processus zygomaticus of the maxilla. It may even 
extend into the maxillary border of the zygomatic bone; thus 
extending the recessus zygomaticus of the sinus maxillaris. 
The ventral wall of the cavity corresponds to the anterior or 
facial surface of the maxilla, and looks ventrolaterally. Part of 
this wall is at times greatly approximated to the dorsal wall 
and base of the sinus, due to a very prominent fossa canina. 
Occasionally the whole ventral wall bulges markedly into the 
savity of the sinus. 
The dorsal wall of the sinus corresponds to the infratemporal 
surface of the maxilla. It is a thin plate of bone, also forming 
the ventral boundary of the infratemporal and the pterygo- 
palatine fossee. This wall is usually the thickest of the sinus 
walls—it is, however, occasionally extremely thin (the processus 
alveolaris being recognized as the floor of the cavity and not as 
a wall). 
The base or median wall is directed towards the cavum nasi. 
It presents a very irregular orifice—hiatus maxillaris, in the dis- 
articulated bone. In the articulated skull this opening is partly 
filled in by the pars perpendicularis of the palate bone, the 
processus uncinatus of the ethmoid bone, the processus max- 
illaris of the inferior nasal concha, and a portion of the lacrimal 
bone. In the undissected state this irregular aperture tormed 
by these bones is rounded by mucous membrane, which is con- 
tinued into the sinus maxillaris from the cavum nasi. This 
rounded opening—ostium maxillare, may be duplicated; and such 
duplication must not be confused with the ostium maxillare 
accessorium, which is a direct passageway between the sinus 
and the cavum nasi. The ostium or ostia maxillaria establish a 
communication between the sinus and the infundibulum eth- 
moidale. The medial wall immediately inferior to the attach- 
ment of the concha nasalis inferior is very thin and is easily 
punctured in this region. This wall also forms the lateral bound- 
ary of the cavum nasi and often markedly encroaches upon the 
cavity of the sinus maxillaris, which greatly influences its size. 
