OSSIFICATION OF THE BONES OF THE HEAD. 



73 



sphenoid, lachrymal, superior maxillary and palate bones. The anterior and middle, 

 the larger and more numerous, open into the middle, the posterior into the superior 

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

 the frontal bone over the root of the nose. They extend outwards from behind the 

 glabella to a variable distance over the orbit, being separated from each other by a 

 thin bony septum. They open on each side into the middle meatus of the nose 

 through the inf undibulum. The sphenoidal sinuses occupy the body of the sphenoid, 

 being formed in connection with the sphenoidal spongy bones. They are separated 

 by a median septum, and open into the spheno-ethmoidal recess. The maxillary 

 sinus has been described in connection with the superior maxillary bone ; it opens by 

 a small aperture into the middle meatus. 



The air-spaces of the temporal bone (tympanum and mastoid cells) are described in connec- 

 tion with the Anatomy of the Ear in Vol. III. 



OSSIFICATION OF THE BONES OF THE HEAD. 



The ossification of the bones of the base of the cranium takes place for the most part in car- 

 tilage, and in each case proceeds from several centres which represent distinct bones in lower 

 vertebrate forms ; the bones of the roof are simpler in their development, and originate in 



B 



Fig. 74. OSSIFICATION OP THE OCCIPITAL 

 BONE. (R. Quain.) 



A, in a foetus of 10 weeks (from Meckel) ; 

 a, tabular part ; 1 & 2, lower and upper pairs 

 of centres ; b, lower part or basilar and con- 

 dylar portions : ossific centres are seen in the 

 condylar portions. 



B, occipital bone at birth ; a, tabular 

 part, in which the four centres have become 

 united into one, leaving fissures between 

 them ; b, b, condylar portions ; c, basilar 

 portion. 



membrane ; those of the face are also deposited in membrane, with the exception of the 

 inferior turbinate bone and a small part of the lower jaw. In the expanded tabular bones the 

 ossification spreads outwards from the centres, and the marginal portions, in the earlier stages, 

 present more or less the form of radiated fibres or spicula. At birth the sutural edges, and 

 especially the angles, are incomplete, the bones being united and the interspaces filled up by 

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

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



The occipital bone at birth consists of four separate pieces a basilar, a tabular, and two 

 condylar parts, united by intervening cartilage. The lines of junction of the basilar and 

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

 and tabular parts extend outwards from the posterior margin of the foramen magnum. The 

 basilar (Tjaswccipital) and condylar parts (cxoccipitalii) arise each from one osseous nucleus, 

 which appears from the eighth to the tenth week. 1 In the tabular part there appear, a few 

 days earlier, usually four nuclei, an upper and a lower pair ; these speedily unite, but leave 

 fissures running in from the upper and lateral angles, which remain for some time after 

 birth. The upper pair of these differ from the other centres of this bone in being deposited in 

 membrane, and while the lower portion of the tabular part is the proper tupraoecipital element, 

 the upper represents the interparietal bone of many animals ; it occasionally happens that this 

 remains distinct in the human skull, the upper part of the occipital squama being separated 

 from the rest by a suture running transversely from one lateral angle to the other, and by no 

 means unfrequently a partial division exists, by persistence of the lateral fissures, which may 



1 In the descriptions of the mode of ossification of the bones, weeks and months refer always to 

 periods of foetal life. 



