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The large facial bones that surround the nasal cavity - the frontal bone, the maxilla, the sphenoid and ethmoid bones - are hollow to a greater or lesser extent. The hollow spaces in these bones contain the paranasal sinuses, which in the healthy living body are filled with air. The paranasal sinuses all communicate with the nasal cavity.

To see the sinus cavities we’ll look at a skull in which part of the bone that overlies each sinus has been removed. Here’s the cavity for the right frontal sinus. There’s a left one too, on the other side of this partition.

The frontal sinus extends upward, behind the lower part of the forehead, and also to a variable extent backwards, between the roof of the orbit and the floor of the anterior cranial fossa. Here’s the cavity for the right maxillary sinus, also known as the maxillary antrum.

It extends backwards to the part of the maxilla that borders the pterygo-maxillary fissure. It extends downwards almost to the root of the upper molar and premolar teeth. The medial wall of the maxillary sinus is also the lateral wall of the nasal cavity. Its roof forms a large part of the floor of the orbit.

The sphenoid sinuses occupy the central part of the sphenoid bone. This opening has been made to show the right sphenoid sinus. To see it better we’ll look at the skull divided in the midline. Here’s the right sphenoid sinus again.

Above the sphenoid sinus is the floor of the anterior cranial fossa, and the sella turcica. Behind it is the basilar part of the occipital bone. In front of it is the high part of the nasal cavity. Below it is the roof of the nasopharynx.

Lastly we’ll come round to the front, to ...

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(2.53)

The large facial bones that surround the nasal cavity - the frontal bone, the maxilla, the sphenoid and ethmoid bones - are hollow to a greater or lesser extent. The hollow spaces in these bones contain the paranasal sinuses, which in the healthy living body are filled with air. The paranasal sinuses all communicate with the nasal cavity.

To see the sinus cavities we’ll look at a skull in which part of the bone that overlies each sinus has been removed. Here’s the cavity for the right frontal sinus. There’s a left one too, on the other side of this partition.

The frontal sinus extends upward, behind the lower part of the forehead, and also to a variable extent backwards, between the roof of the orbit and the floor of the anterior cranial fossa. Here’s the cavity for the right maxillary sinus, also known as the maxillary antrum.

It extends backwards to the part of the maxilla that borders the pterygo-maxillary fissure. It extends downwards almost to the root of the upper molar and premolar teeth. The medial wall of the maxillary sinus is also the lateral wall of the nasal cavity. Its roof forms a large part of the floor of the orbit.

The sphenoid sinuses occupy the central part of the sphenoid bone. This opening has been made to show the right sphenoid sinus. To see it better we’ll look at the skull divided in the midline. Here’s the right sphenoid sinus again.

Above the sphenoid sinus is the floor of the anterior cranial fossa, and the sella turcica. Behind it is the basilar part of the occipital bone. In front of it is the high part of the nasal cavity. Below it is the roof of the nasopharynx.

Lastly we’ll come round to the front, to look at the collection of small cavities that contain the ethmoid air cells, collectively referred to as the ethmoid sinus. These extend from just behind the naso-lacrimal duct, all the way back along the medial wall of the orbit. As we’ve seen already, the ethmoid air cells lie between the medial wall of the orbit, and the lateral wall of the upper part of the nasal cavity.

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