THE HEAD. 55 



free, concave, and sharp, prolonged in front and al)ove by the median crest of 

 the frontal bone, and confounded behind with the middle portion of the inferior 

 sphenoid. The utferior border is continuous with the cartilac^inous plate which 

 separates the nasal cavities. When this plate becomes ossified, which is not 

 unfrequent, it is impossible to discover the point where it begins or the ethmoid 

 bone terminates. The middle septum of the nose has been considered, and 

 justly, as a prolongation of the perpendicular plate (or hnniiKi) of this bone. 

 The anterior border is consolidated with the vertical septum which separates the 

 frontal sinuses. The posterior border is joined al)ove to the median plate which 

 divides the sphenoidal sinuses into two compartments. Below, it is fixed in the 

 groove of the vomer, and soon becomes confounded with that bone, which is 

 itself consolidated with the inferior sphenoid. 



Lateral Masses of the Ethmoid Bone. — These are two large pyriform 

 tuberosities placed on each side of the perpendicular lamina, and offering for 

 study a middle portion, a base, and a summit. Each of these is formed by an 

 assemblage of numerous, extremely thin, osseous plates, curved into small and 

 very fragile convolutions. These, elongated from above to below, become 

 longer as they are more anterior ; they are attached by their superior extremities 

 to the transverse plate which separates the cranium from the nasal cavities, and 

 by one of their borders to a thin leaf of bone which envelops the lateral masses 

 outwardly. They have received the name of the ethmoidal volutes (or cells). 



Middle portion. — This should be studied externally and internally 



The external surface of each ethmoidal mass is divided into two sections : 

 an internal, making part of the nasal cavities ; the other, external, concurs in 

 forming the walls of the frontal and maxillary sinuses. The first, the least 

 extensive, is almost plane ; parallel to the perpendicular lamina, it is isolated 

 from it by the narrow space which forms the bottom of the nasal cavities ; it 

 presents several openings which separate the most superficial cells, and join the 

 internal canals to be hereafter noticed. The second, very extensive and convex, 

 looks outwards in front and behind, and is covered by an osseous plate traced 

 with shallow furrows, which correspond internally with the small crests to which 

 the cells are attached. This lamella is prolonged, inferiorly, a little beneath the 

 inferior extremity of these latter, and turns outwards to articulate with the 

 palate and superior maxillary bones ; superiorly, it coalesces with the sphenoid 

 and the orbital portion of the frontal bone. 



Infernidhj, the lateral masses are hollowed from above to below by extremely 

 diverging canals, which open inferiorly into the nasal cavities, and separate the 

 cells from one another. The latter are so incurvated that the internal cells 

 communicate with each other. There are some, however, which are completely 

 closed : the anterior, or great cell, is frequently so.^ 



Base. — The base of each lateral mass looks upwards, and is formed by the 

 transverse septum between the cranium and the nasal cavities. This septiun is 

 perforated by openings which give passage to the ethmoidal nerves ; it is named 

 the cribriform plate of the ethmoid bone. It is concave on the superior surface, 

 which constitutes the ethmoidal fossa, and convex on the opposite face, where 

 attachment is given to the superior extremities of the cells. It is consolidated 

 internally with the perpendicular plate ; the other points of its circumference 

 are attached to the sphenoid bone, and to the transverse ridge on the internal 

 face of the frontal bone. 



' It is not rare to find it opening into the superior maxillary sinus. 



