THE BONES OF THE HEAD 139 



The outer wall is characterized by great irregularity, and is 

 formed by seven bones, in the following order, as nearly as possible, 

 from before backwards : (i) the nasal ; (2) the nasal process of 

 the superior maxilla ; (3) the lachrymal ; (4) the internal surface 

 of the lateral mass of the ethmoid, presenting the superior and 

 inferior turbinate processes (superior and middle spongy bones) ; 

 (5) the inferior turbinate or spongy bone which lies below the last 

 named ; (6) the perpendicular plate of the palate bone, together 

 with parts of its orbital and sphenoidal processes ; and (7) the 

 internal pterygoid plate of the pterygoid process of the sphenoid. 



The bulging projections on this wall are produced by the superior 

 and inferior turbinate processes of the ethmoid and the inferior 

 turbinate bone, in this order from above downwards, and the deep 

 channels thereby formed are known as the tneatH. These are 

 three in number — superior, middle, and inferior. The superior 

 meatus is situated towards the back part of the outer wal', where 

 it lies between the superior and inferior turbinate processes of the 

 ethmoid. It is comparatively short, and is directed obliquely 

 forwards and upwards. The posterior ethmoidal cells open into 

 it anteriorly, and the spheno-palatine foramen posteriorly. The 

 middle meatus, which is longer than the superior, lies between the 

 inferior turbinate process of the ethmoid and the inferior turbinate 

 bone. At its anterior part it turns upwards, and is continued into 

 the passage known as the infundibulum, which communicates with 

 the frontal air sinus of its own side. The ascending part also 

 communicates with the anterior ethmoidal cells. The middle 

 portion communicates with the middle ethmoidal cells, and pre- 

 sents the opening of the antrum of Highmore. The inferior meatus, 

 which is the longest of the three, lies between the inferior turbinate 

 bone and the floor of the nasal fossa. Near its anterior part 

 is the lower orifice of the lachrymal canal, which lodges the nasal 

 duct. 



The Air Sinuses. — These are hollow cavities lined with mucous 

 membrane, which are contained within the following bones : the 

 frontal, sphenoid, ethmoid, superior maxillae, and mastoid portions 

 of the temporals. They commimicate directly with the nasal fossae, 

 except the mastoid cells, which at either side are in communica- 

 tion with the tympanum, that in turn being connected by means of 

 the Eustachian tube with the naso-pharynx, at a point external to 

 the posterior naris. The maxillary air sinus or antrum of Highmore 

 appears about the fourth month of intra-uterine life, but the 

 other air sinuses do not appear until chUdhood, and they, do 

 not show much development until the period of puberty (see 

 the individual bones). In old age they all tend to become 

 enlarged. 



The frontal sinus (through means of the infundibulum) and the 

 anterior ethmoidal cells open into the ascending front part of the 

 middle meatus. The middle ethmoidal cells and the maxillary 

 sinus open into the central portion of the middle meatus. The 



