96 A MANUAL OF ANATOMY 



fossa, is formed by the lower border of the inferior turbinate 

 process. Anteriorly it articulates with the superior turbinate 

 crest of the superior maxilla, and posteriorly with the ethmoidal 

 or superior turbinate cre^t of the palate bone. The anterior border, 

 like the superior, presents depressions, which form air cells when 

 the lachrymal and nasal process of the superior maxilla are in 

 position. This border projects slightly in advance of the front 

 of the OS planum, and from the lower part of this projecting 

 portion there springs the uncinate process. This is a long, thin, 

 curved plate which extends downwards, backwards, and slightly 

 outwards into the anterior part of the middle meatus. In its 

 course it crosses the opening of the antrum of Highmore in the 

 superior maxilla, and thus forms part of the inner wall of that 

 air sinus. The lower border of the process presents two spur-like 

 projections, between which the border is markedly concave. The 

 posterior terminal spur articulates with the ethmoidal process of 

 the inferior turbinate bone. The posterior border of the, lateral 

 mass presents a few depressions, closed by the sphenoidal spongy 

 bone and orbital process of the palate bone, which latter process 

 becomes ankylosed with it about the fourth year. 



The ethmoidal cells are contained within each lateral mass, and 

 are lined with mucous membrane, which is continuous with that of 

 the nose. They are arranged in three sets — anterior, middle, and 

 })osterior. The anterior ethmoidal cells, along with the frontal sinus 

 of the same side, open by a common passage, already described 

 as the infundibulum, into the ascending front part of the middle 

 meatus ; the middle ethmoidal cells open into the horizontal part 

 of the middle meatus ; and the posterior ethmoidal cells open into 

 the superior meatus. 



The bone receives its blood - supply from the anterior and 

 posterior ethmoidal branches of the ophthalmic, and the spheno- 

 palatine branch of the internal maxillary. 



Articulations. — The ethmoid articulates with fifteen bones, as 

 follows : (i) frontal (nasal spine and orbital plates) ; (2) sphenoid 

 (ethmoidal spine and sphenoidal crest) ; (3) two sphenoidal spongy 

 bones ; (4) two nasal bones (nasal crest) ; (5) vomer (cleft between 

 alse) ; (6) two palate bones (ethmoidal or superior turbinate crests 

 and orbital processes) ; (7) two lachrymals (upper part of internal 

 surface) ; (8) two superior maxillae (nasal processes, orbital plates, 

 and opening of each antrum) ; and (9) two inferior turbinates 

 (ethmoidal processes). 



Structure. — The lateral masses are excavated into many thin- 

 walled air cells, and the crista galli contains a small amount of 

 cancellated tissue. 



Ossification. — The ethmoid is developed in cartilage from three centres. 

 Two of these appear in the fifth month of intra uterine life, one in each os 

 planum, from which ossification extends into the superior and inferior turbinate 

 processes. At birth the lateral masses are ossiiied, but the perpendicular 

 plate and crista galli are cartilaginous. In the first year a centre appears 

 at the base of the crista galli, and from this ossification extends upwards into 



