186 HEALTH AND DISEASE 



Ethmoid or Sieve Bone. The ethmoid bone is situated in front 

 of the sphenoid, and forms the lower part of the division separating the 

 cranium from the face. It consists of two lateral halves, separated by 

 a perpendicular plate (the lamina). Each half in front consists of a 

 number of thin fragile plates of bone, rolled up into small scrolls (ethmoidal 

 cells), and attached to the cribriform plates, i.e. two bony plates having 

 a number of small holes in them, by which the olfactory nerves escape 

 from the cranium into the nostrils. At the sides it throws up two wing- 

 like processes, which articulate with the frontal bones. 



Frontal Bones. - - These bones form a portion of the inferior wall 

 of the cranium, as well as that part of the face corresponding to the 

 forehead. They are situated between the parietal bones above and the 

 nasal and lachrymal bones below, and have union with several other bones 

 of the cranium and face. Each of the frontal bones assists in forming 

 an irregular cavity of considerable extent (frontal sinus), which con- 

 tains air and communicates with the nostril. These frontal sinuses are 

 lined by mucous membrane. They give lightness to the head, and in 

 " nasal gleet " sometimes require to be opened in order to give exit to 

 the pus which accumulates within them. They are very small in early 

 life, but enlarge as age advances, and are separated one from the other 

 by a bony partition. 



BONES OF THE FACE 



Nasal Bones. These bones form the anterior part of the face below, 

 and are situated beneath the frontal bones, and between the lachrymal 

 and the superior and anterior maxillary bones. They are the slender 

 bones commencing above by a broad extremity and ending below in a 

 pointed process (nasal peak). They form the front boundary wall of 

 the nasal cavities. 



Superior Maxillary Bones. - - The upper jaw - bones are situated 

 on the side of the face, and join together by means of a flattened plate 

 (palatine process) in the centre of the roof of the mouth, a large portion 

 of which they form. They also form the floor and sides of the nostrils, 

 and the sockets, or alveoli, into which the fangs of the upper grinders 

 or " molar" teeth are implanted. Like the frontal bones, they are hollowed 

 out into sinuses, which sometimes become diseased as the result of direct 

 injury to the jaw, or to disease extending from the fangs of the teeth. 

 In these cases they become filled with matter to which exit has to be 

 given by an operation. 



Anterior Maxillary Bones. - - These bones are situated at the 



