BONES OF THE HEAD.— OCCIPITAL. 31 



tody, and imperfect, union of the tirst, second, and third pieces of the body, wliile the 

 third and fourth are united. 



J), example of a sternum at birth with an unusual number of ossific centres, six 

 in the manubrium, 1', wuicli is very uncommon ; two pairs in the lower pieces of 

 the body 3' & 4', which is not unusual ; 2, the single centre of the fir&t piece of the 

 body. 



E, example of the perforated sternum, which probably depends upon the imperfect 

 union of the pairs of ossific nuclei shown in D in the lower part of the body ; this figure 

 also shows two episternal bones or granules, * *. C, and E, are reduced below the size of 

 nature. 



a single centre in the presternum. - Tlie next centre appears at the seventh 

 month in the upper segment of the mesostenium, and ossification foUows in the 

 nest two segments shortly before birth. In the lower segments, ossification 

 begins in the first year or later, and in the metasternum not before the sixth 

 year, and often much later. In the presternum occasionally two centres of ossi- 

 fication appear, one above the other. In the upper segment of the mesostemuna 

 the centre is most commonly single, but in each of the following segments there 

 are frequently two placed one on each side of the middle line. The lower seg- 

 ments of the mesostemum unite together after puberty, but the upper one often 

 remains separate till after the twenty-fifth year. The metasternum is united to 

 the mesostemum in middle life, the presternum in more advanced life. Tlie 

 bony parts formed from the lateral centres of the lower segments of the meso- 

 stemum, as well as of the metasternum, not unfrequently remain separate for a 

 considerable time, and occasionally, by defect of ossification or non-union across 

 the middle line, have a pennanent median apertm-e in the bone — the so-called 

 foramen stcrnalc. The metasternum is sometimes bifid, and in rare cases the 

 mesostemum has been found di\'ided to a greater or less extent from below 

 iipwards, constituting the malfonnation of fissura sterni. and connected in some 

 instances with ectopia cordis. (See the case of E. Groux, described by Allen 

 Thomson in the Glasgow Med. Joiu-nal, 1858.) 



III.— THE BONES OP 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 almost immovably united together by lines and narrow 

 surfaces, more or less uneven, termed sutures. The skull is divided by 

 anatomists into two parts, the cranmm and the face. The cranium 

 protects 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, U\o parietat, the 

 frontal, two temporal, the sphenoid, and the ethmoid. The face is com- 

 posed of fourteen bones, of which twelve are in pairs, viz. : the sitperior 

 maxiUary, malar, nasal, jmlafe, lachrymal, and inferior turbinated hones ; 

 and two single, viz., the vomer, and the inferior maxilla. There is also 

 suspended by ligaments from the under surface of the cranium, the 

 hyoid bone, which may be classed with the bones of the head. 



THE OCCIPITAL BONlS 



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

 cranium. In general form it is rhomboidal, and through its lower and 

 anterior part passes a large oval foramen, forming the communication 

 between the cranium and spinal canal. The portion of the bone behind 

 the foramen is talndar, the narrower part in front forms a thick mass 

 named hasilar process, and the parts on the sides of the foramen, bearing 



