78 PRACTICAL ANATOMY. 



The skull differs in form in different races, in the sexes, 

 and at the various ages or periods of life. 



Age. The skull in infancy presents an immensely 

 large cranial and a small facial portion, the facial portion 

 being but one-eighth as large as the cranial. The air- 

 sinuses, such as the frontal, maxillary, and sphenoidal, 

 are entirely undeveloped ; the diploic structures are not 

 formed until about the tenth year. The alveolar proc- 

 esses are but little developed, containing, however, the 

 developing deciduous teeth and the germs of the perma- 

 nent set. None of the articulations are firm, although 

 such sutures as the frontal and that between the two halves 

 of the inferior maxillary are beginning to give evidences 

 of ossification. Large gaps are seen at different parts of 

 the skull, notably at the anterior and posterior part of 

 the superior surface, constituting the fontanelles. The 

 original centres of ossification are exceedingly prominent, 

 as is seen in the frontal and parietal eminences. These 

 eminences or bosses serve as " fenders " to the skull of 

 the infant, protecting the structures within from serious 

 injury. In the adult female, the cranial portion of the 

 skull predominates, constituting about three-fifths, the 

 facial portion constituting but two-fifths of the skull. 

 In the adult male, the cranial and facial portions are 

 nearly equal. This is due to the greater development 

 in the male of the large facial air-sinuses, and the nor- 

 mally larger formation of the facial bones. In old age 

 the skull undergoes a retrograde change ; the teeth fall 

 out, the alveolar processes become absorbed, the sym- 

 physis menti becomes elevated and protrudes owing 

 to the greater obliquity of the angle of the inferior 

 maxillary, the air-sinuses diminish in size, and the cra- 

 nial portion again largely predominates ; in fact, the 

 entire skull diminishes in size, the cranial as well as 



