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ANATOMY AND PHYSIOLOGY                           9

maxillary sinus, the frontal sinus and the anterior ethmoidal cells, all of which
communicate with the middle meatus. The posterior group consists of the
posterior ethmoidal cells and the sphenoidal sinus Communicating respectively
with the superior meatus and the spheno-ethmoidal recess.

The maxillary, sinus is also known as the maxillary antrum, or, simply, the
antrum. It exists at birth as a small but definite cavity adjacent to the middle
meatus, and it enlarges gradually to reach its maximum dimensions about the
twenty-first year with the eruption of the upper wisdom tooth (Figs. 9, 10).

Fig. 9. Coronal section of the right maxilla
during the period of the first dentition; the
small maxillary sinus lies medially to the infra-
orbital canal; the maxilla consists largely of
cancellous bone. 1, Infra-orbital canal; 2, Right
maxillary sinus; 3, Second molar tooth.

Fig. 10. Coronal section of the right maxilla of
an adult showing the fully developed maxillary
sinus. The floor of the sinus is on a lower plane
than that of the nasal floor; the molar fang
projects into the cavity of the sinus. I, Infra-
orbital canal; 2, Right maxillary sinus; 3, First
molar tooth; 4, Maxillo-ethmoidal cell.

The sinus expands in the maxilla during the eruption of the primary dentition
until it reaches the level of the floor of the nasal cavity about the seventh year.
In adult life it is somewhat pyramidal in shape, its roof being formed by the
floor of the orbit; its floor being in close proximity to the roots of the second
dentition; its posterior wall lying in relation to the infratemporal and
pterygopalatine fossae; its medial wall adjoining the lateral wall of the nasal
cavity; and its anterolateral walls being superficial. The opening into the
middle meatus, the maxillary ostium, is near the upper part of the cavity of the
sinus, and is thus unfavourably placed for drainage. There may be one or
more accessory ostia posterior to the main one.

The frontal sinus is rudimentary at birth, being represented by a small
upward prolongation from the anterior end of the middle meatus, the naso-
frontal duct During childhood this duct enlarges upwards to reach the level