SKULL. OCCIPITAL BONE. 29 



a considerable time ; the second segment has rarely more than one 

 nucleus. Additional nuclei occasionally appear between some of the 



segments. 



PERIODS OF OSSIFICATION OF THE RIBS AND STERNUM. 



Ill the ribs 



The principal centres appear in the 7th or 8th week (foetal life). 



The epiphyses appear from the 16th to the 20th year. 



The shaft and epiphyses unite about the 25th year. 

 In the sternum 



Ossification in the first segment appears in the 6th or 7th month (foetal life). 



Ossification in the second segment appears in the 7th or 8th month (foatal life). 



Ossification in the third and fourth segments appears shortly before birth. 



Ossification in the fifth segments appears in the 1st year or later. 



Ossification in the sixth or ensiform segment appears very variously from the 6th 

 to the 1 5th year, or later. 



The lower segments of the body unite after puberty. 



The upper segments of the body unite from the 25th to the 30th year. 



The body and ensiform segment generally unite in middle age. 



The manubrium and body unite in old age. 



III. THE BONES OF THE SKULL. 



The skull is of a spheroidal figure, compressed on the sides, broader 

 behind than before, and supported on the vertebral column. It is formed of 

 a number of bones, all of which, 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 cranium and the face. The cranium protects the brain ; the face 

 surrounds the mouth and nasal passages, and completes with the cranium the 

 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 turbinated bones ; and two single, viz., the vomer, and the inferior 

 maxilla. 



THE OCCIPJTAL BONE. 



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

 Its general form is that of a curved lozenge concave superiorly, through 

 which, in its lower and anterior part, passes a large oval foramen, form- 

 ing the communication between the cranium and spinal canal. The bone 

 thus presents four borders and four angles, which receive names according 

 to their position. The portion of the bone behind the foramen is tabular, 

 that in front of the foramen forms a thick mass named basilar process, and 

 the parts on the sides of the foramen, bearing the condyles or articulating 

 processes by which the head is supported on the first vertebra, are dis- 

 tinguished as the condyloid portions. 



The occipital bone articulates by its two superior borders with the parietal 

 bones, and by its two inferior borders with the temporal bones, 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 con- 

 tinuous osseous* substance. 



* Hence Soemmerring described the occipital and sphenoid as a single bone under the 

 name spheno-occipital, or basilar. 



