90 



THE SKELETON 



the horizontal plate of the palate bone which completes the hard palate. The medial border 

 joins with its fellow to form the nasal crest upon which the vomer is received. The more 

 elevated anterior portion of this border is known as the incisor crest, and is continued forward 

 into the anterior nasal spine. The septal cartilage of the nose rests on its summit and the anterior 

 extremity of the vomer lies immediately behind it. At the side of the incisor crest is seen the 

 upper aperture of the canal leading from the nose to the mouth (Stenson's canal), which in its 

 course downward becomes a groove by a deficiency of its medial wall. Thus when the two bones 

 are articulated a canal is formed (incisive) with the lower ends of two canals opening into it. 



The alevolar process is crescentic in shape, spongy in texture, and presents 

 cavities [alveoli dentales] in which the upper teeth are lodged. When complete 



there are eight tooth-cavities (alveoli), wdth wide mouths, gradually narrowing 

 as the}^ pass into the substance of the bone, and forming exact impressions of the 

 corresponding fangs of the teeth. The pit for the canine tooth is the deepest; 

 those for the molars are the widest, and present subdivisions. Along the lateral 

 aspect of the alveolar process the buccinator arises as far forward as the first molar 

 tooth. 



The maxillary sinus or antrum of Highmore, as the air-chamber occupying 

 the body of the bone is called, is somewhat pyramidal in shape, the base being 

 represented by the nasal or medial surface, and the apex corresponding to the 

 zygomatic process. In addition it has four walls: the superior "is formed by the 

 orbital plate, and the inferior by the alveolar ridge. The anterior wall corre- 

 sponds to the anterior surface of the maxilla, and the posterior is formed by the 

 infratemporal surface. The medial boundary or base presents a very irregular 



Fig. 113. — The Maxilla at Birth. 



Premaxillary portion 



Outer view 



Inferior view 



Inner view 



orifice at its posterior part; this is partially filled in by the vertical plate of the 

 palate bone, the uncinate process of the ethmoid, the maxillary process of the 

 inferior nasal concha, and a small portion of the lacrimal bone. Even when these 

 bonos are in their relative positions, the orifice is very irregular in shape, and 

 requires the mucous membrane to form the definite rounded aperture (or apertures, 

 for they are often multiple) known as the opening of the sinus through which 

 the cavity communicates with the middle meatus of the nose. 



The cavity of the sinus varies considerably in size and shape. In the young, it is small and 

 the walls are thick: as life advances it enlarges at the expense of its walls, and in old age they are 

 oft(!n fxtrcmcly thin, so that occasionally the cavity extends even into the substance of the 

 zygomatic bone. The floor of the sinus is usually very imeven, due to prominences corre- 

 sponding to the roots of the molar teeth. In most cases the bone separating the teeth from 

 the sinus is very thin, and in some cases the roots project into it. The teeth which come into 

 closest relationship with the sinus are the first and second molars, but the sockets of any of 

 the teeth lodged in the maxilla may, under diseased conditions, communicate with it. As a 

 rule, the cavity of the sinus is single, but occasionally specimens are seen in which it is divided 

 by bony septa into chambers, and it is not uncommon to find recesses separated by bony 

 processes. The roof of the sinus presents near its anterior aspect what appears to be a thick 

 rib of bone; this is hollow and corresjionds to the infra-oriiital canal. 



The most satisfactory method of studying the relation of the bones closing in the base of 

 the antrum is to cut away the lat(!ral wall of the cavity (see fig. 128). 



Blood-supply. — The niaxilhi is a vrjry vascular bone and its arteries are numerous and 

 large. 'I'hey are derived from tin; infra-orbital, alveolar, descending palatine, spheno-pala- 

 tine, ethmoidal, frontal, nasal, and facial vessels. 



Articulations. —With the frontal, nasal, lacrimal, ethmoid, palate, vomer, zygomatic, 

 inferior nasal concha and its fellow of the opposite side. Occasionally it articulates with the 

 great wing, and the pterygoid process, of the sphenoid. 



