144 



SPECIAL ANATOMY OF THE SKELETON 



closed by the growth and extension of the bones which surround them, but sometimes they are 

 the sites of separate ossific centres which develop into Wormian bones. The posterior and 

 lateral fontanelles are obliterated within a month or two after birth, but the anterior is not com- 

 pletely closed until the first half of the second year; sometimes it remains open beyond the second 

 year, a condition which is usually seen in rhachitis, and is due to malnutrition. A knowledge of 

 the shape and position of the fontanelles is of service to the accoucheur in enabling him to deter- 

 mine which part of the fetal head is presenting during parturition. 



The small size of the face at birth is mainly accounted for by the rudimentary condition of the 

 jaws, the noneruption of the teeth, and the small size of the maxillary air sinuses and nasal 

 cavities. At birth the nasal cavities lie almost entirely between the orbits, and the lower border 

 of the anterior nasal aperture is only a little below the level of the orbital floor. With the erup- 

 tion of the milk teeth there is an enlargement of the face and jaws, and these changes are still 

 more marked after the second dentition. 



FIG. 111. Skull at birth, showing the anterior 

 and posterior fontanelles. 



FIG. 112. The lateral fontanelles. 



The skull grows rapidly from birth to the seventh year, by which time the foramen magnum 

 and petrous parts of the temporals have reached their full size and the orbital cavities are only 

 a little smaller than those of the adult. Growth is slow from the seventh year until the approach 

 of puberty, when a second period of activity takes place; this consists of an increase in all direc- 

 tions, but it is especially marked in the frontal and facial regions, where it is associated with the 

 development of the air sinuses. 



Obliteration of the Sutures.. Obliteration of the sutures of the vault takes place as age 

 advances, usually beginning during the fourth decade of life and first becoming manifest on the 

 inner surface, appearing externally about ten years later; the posterior part of the sagittal suture 

 is usually the first to become obliterated, next the coronal, and then the lambdoid. The most 

 striking feature of an old skull is the marked diminution in the size of the jaws consequent on 

 the loss of the teeth and the absorption of the alveolar processes, thus reducing the facial height 

 and altering the mandibular angles. 



Differences in the Skull Due to Sex. Until the age of puberty little difference exists be- 

 tween the male and the female skull. The skull of an adult female is, as a rule, lighter and 

 smaller. While the cranial capacity of white males averages 1560 c.c., that of females is nearly 

 200 c.c. less. The female skull has thinner walls, its ridges for muscle attachment are less 

 strongly marked, the superciliary ridges, glabella, and mastoid processes are less prominent, and 

 the corresponding air sinuses are smaller. The upper margin of the orbit is sharper, the frontal 

 and parietal eminences are more prominent, and the vault is somewhat flattened as compared 

 with the male skull. The contour of the face is more rounded, the facial bones are smoother, 

 and the jaws and teeth are smaller. 



No single structural characteristic, however, serves to determine the sex, and the features 

 enumerated above can guide in the examination only when they are sufficiently pronounced to 

 justify a probable diagnosis. 1 



i See P. J. Mobius: Ueber die Verschiedenheit miinnlicher und weiblicher Schiidel. Arehiv fur Anthropologie, 

 1907, N. F., vol. vi. 



