OSSIFICATION OF THE BONES OF THE HEAD. Q7 



tissue in their vicinity. The ethmoidal sinuses consist of several irregular 

 spaces occupying the lateral masses of the ethmoid bone. The 

 anterior open into the infundibulum, the posterior into the superior 

 meatus. The frontal sinuses are placed between the outer and inner 

 tables of the frontal bono over the roof of the nose. They extend out- 

 wards from behind the glabella to a variable distance over the orbit 

 being separated from each other by a thin bony septum. Tliey open 

 into the middle meatus of the nose through tlie infundibula. The 

 sjilienoidal sinuses occupy the anterior part of the body of the sphenoid 

 bone, being formed in connection with the sphenoidal spongy bones. 

 They are separated by a median septum, and open anteriorly behind 

 and above the superior meatus of the nose. 



The maxillary sin as is of an irregular pyramidal form; its apex 

 points to the malar tuberosity ; its inner wall, which separates it from 

 the nasal cavity, is formed by the maxillary, palate, and inferior 

 turbinated bones, and the uncinate process of the ethmoid ; its other 

 sides are formed by the superior maxillary bone, an irregular gap or 

 deficiency being left between the uncinate process and the inferior 

 turbinated bone, by which the sinus opens into the middle meatus. 

 The alveolus of one of the molar teeth generally forms a marked pro- 

 jection in the floor of the sinus, at its outer part. 



OSSIFICATION OF THE BONES OF THE HEAD. 



Some of these bones originate in cartilage, and others in membrane, or un- 

 cliondiified formative tissue. 



To the first set belong chiefly the bones of the base of the cranium, viz.. the 

 occipital, sphenoid, ethmoid, petro-mastoid, and inferior turbinated, together with 

 the small bones of the ear, the styloid process, and hyoid bone. All the other 

 bones of the cranium and face originate in membrane or formative tissue. The 

 parts of the bones which are median or central — the basi-occipital. basi-sphenoid, 

 pre-sphenoid, and mesethmoid — ^arise either from one median centre each, or from 

 more numerous centres, which coalesce into one at an early period. The other 

 bones or then* parts are all in paii's, although some of them, as the frontal, ex- 

 occipital, ethmo-turbinal, alisphenoid, orbito-sphenoid, and lower jaw finally 

 coalesce with each other or with a median portion. 



In the tabular bones ossification usually spreads outwards fi'om cei"tain centres, 

 and the marginal portions, in the earlier stages, present more or less the form of 

 radiated fibres or spicula. The sutured edges, and esijecially the angles of the 

 "bones, are incomplete, the bones being united together at theii- margins by fibrous 

 tissue. The diploe and sinuses are at first absent, some of the latter arising early 

 in life, and others being fonned at a comparatively late but variable period. 



The occipital bone. This bone at birth, and for some time afterwards, con- 

 sists of four separate pieces — a basilar, a tabular, and two condylar parts, united 

 together by intervening cartilage. The lines of junction of the basilar and con- 

 ■dylar parts pass through the condyles near their anterior extremities ; those of 

 the condylar and tabular parts extend outwards from the jjosterior margin of the 

 foramen magnum. The basilar and condylar paiiis arise each from one osseous 

 ■nucleus, which appears from the seventh to the eighth week. A few days earlier 

 there appear in the tabular part usually four nuclei, placed in pairs above and 

 below the occipital protuberance. These nuclei .speedily unite to form a single 

 thin tabular mass. Four fissures, however, remain, two median and two lateral 

 T.Tinning in from the margins of the bone, and marking the limits of the original 

 pieces. These fissures, especially the lateral, sometimes remain visible for several 

 years, simulating fractures. The largest ossa Wormii, when symmetrical, may arise 

 from the persistence of the upper and lateral fissui-es. The osseous union of the 



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