THE BONES OF THE HEAD. 81 



remain separate for a considerable time, and occasionally, by defect of ossification or non- 

 union across the middle line, leave the permanent median aperture referred to on p. 25. 



The ossification of the ribs begins in cartilage posteriorly about the eighth or 

 ninth week of foetal life, and extends rapidly forwards, so as to reach the permanent 

 cartilage about the fourth month. After puberty the centres of two small epiphyses appear 

 in the cartilage of the head and tuberosity. These become united with the main bone by the 

 twenty-fifth year. The epiphysis of the tuberosity is wanting in the eleventh and twelfth. 



In the adult the first costal cartilage usually becomes the seat of a superficial ossification, 

 which may proceed so far as to form a complete sheath around it ; and in advanced life the 

 other cartilages are frequently more or less covered by bone, especially on their anterior 

 surface. The tendency to bony deposit is as a rule stronger in the male than in the female, 

 but the age at which the ossification begins, and the extent to which it proceeds, are subject to 

 great variation. The cartilage itself is but seldom ossified. 



III. THE BONES OF THE HEAD. 



The skull, comprising the bones of the head, is of a spheroidal figure, compressed 

 on the sides, broader behind than before, and supported on the vertebral column. 

 All its bones, with the exception of the lower jaw, are immoveably united together 

 by lines and narrow surfaces, more or less uneven, termed sutures. The skull is 

 divided by anatomists into two parts, the cranium and the face. The cranium pro- 

 tects the brain ; the face surrounds the mouth and nasal passages, and completes 

 with the cranium the orbits or cavities for the eyes. The cranium is composed of 

 eight bones, viz. : the occipital, two parietal, the frontal, two temporal, the sphenoid, 

 and the ethmoid. The face is composed of fourteen bones, of which twelve are in 

 pairs, viz. : the superior maxillary, malar, nasal, palate, lachrymal, and inferior 

 iurbinate bones ; and two single, viz. : the vomer, and the inferior maxilla. The 

 hyoid bone, suspended by ligaments from the under surface of the cranium, may also 

 be classed with the bones of the head. 



THE OCCIPITAL BONK. 



The occipital bone is situated at the lower and back part of the cranium. In 

 general form it is flattened and lozenge-shaped, with the longest diameter directed 

 from behind, forwards and downwards. It is much curved, so that one surface is 

 concave and looks forwards and upwards, while the other is convex and looks back- 

 wards and downwards. At the lower and fore part it is pierced by a large oval 

 aperture, the occipital foramen ox foramen magnum, which forms the communication 

 between the cranial cavity and the spinal canal. The portion of the bone behind 

 the foramen is tabular, the narrower part in front forms a thick mass named basilar 

 process, and the parts on the sides of the foramen, bearing the condyles or articu- 

 lating processes by which the head is supported on the atlas, are the condylar 

 portions. 



The two superior borders are deeply serrated, and articulate with the parietal 

 bones in the lambdoid suture. By its two inferior borders, which are uneven but 

 not deeply serrated, it articulates with the mastoid and petrous portions of the 

 temporal bone ; while the extremity of its basilar process is united to the body of 

 the sphenoid, in the young condition by cartilage, but after the age of twenty years 

 by continuous osseous substance. The rhombic form generally given by the meeting 

 of these borders at the four angles is not unfrequently somewhat changed to the 

 octagonal, by the projection of subordinate obtuse angles between the upper and 

 lateral, and between the lateral and lower angles. 



The tabular portion, on Us posterior surface, presents about the centre a promi- 

 nence the external occipital protuberance, from which the superior curved line arches 

 outwards on each side towards the lateral angle of the bone, thus dividing the 



