The Sinus Maxillaris in Man. 331 
The roof of the sinus maxillaris is a very thin plate of bone, 
at times of a papery delicacy. It also forms the floor of the orbit 
and the orbital surface of the maxilla. It is often modeled by a 
ridge formed by the infraorbital canal. In some cases the ridge is 
replaced by a groove which is covered over with the mucous 
membrane of the cavity. At times the roof of the sinus is par- 
tially divided into two plates separated by air cells. Occasionally 
the palate bone aids in forming the roof. 
The floor of the sinus is formed by the processus alveolaris 
of the maxilla. It is by far the thickest of the osseous bound- 
aries of the cavity—the thickness of the floor depending upon 
the degree of hollowing out of the process. In cases where the 
hollowing out has been earried far, the floor of the sinus will bear 
an important relation to some of the teeth and their sockets. The 
floor of the sinus may be thrown into irregular elevations by the 
fangs of the teeth—this depending upon the thickness of the 
layer of spongy bone. This layer varies in thickness in different 
skulls and there may be considerable asymmetry on the twe sides 
of the same skull. The relation of the teeth will be considered 
in a subsequent paragraph. 
THE RELATION OF THE SINUS FLOOR TO THE NASAL FLOOR. 
The relation of the floor of the sinus maxillaris to the floor of 
the nose depends largely upon the degree of hollowing out of the 
processus alveolaris of the maxilla. The degree of arching of the 
pvalatum durum—-thereby affecting the floor of the nose, has also 
some bearing on this relation. When the layer of spongy bone 
is thin, 1. e., the processus alveolaris of the maxilla markedly 
hollowed out, the floor of the sinus is at a level inferior to the 
nasal floor. On the other hand, when the processus alveolaris 
is comparatively thick, the floor of the nose is inferior to the 
sinus floor. Occasionally both floors are in the same plane 
(figs. 8,9 and 11). When the anterior surface of the maxilla and 
the lateral wall of the nose are markedly bulging towards the sinus 
maxillaris, the floor of the nose is, as a rule, inferior to the floor 
of the sinus. It, however, remains that the majority of sinuses 
