234 HUMAN ANATOMY. 



of the skull become still thinner and may be quite absorbed. The thinning of the vault is less 

 marked Occasionally, in extreme age, symmetrical depressions appear m the upper parts ot 

 the parietals behind the vertex. In the latter part of life the frontal smuses enlarge, as the uiner 

 table follows the shrinking brain. In some rare cases the skull grows heavier in old age, owing 

 to an increase in thickness of the inner table. • , „ , u . 



Differences due to Sex.— There is no marked sexual difierence in skulls up to puberty. 

 These characteristics appear during the last stage of grovvth. They may be summed up by 

 saying that the female skull differs less than the male from that of childhood. The parietal and 

 frontal eminences are more prominent ; the superciliary prominences and glabella less marked ; 

 the zygomata mastoid, occipital protuberance, and muscular ridges less developed. The whole 

 structure is li'crhter The face is smaller in proportion to the cranium, owing to the lighter 

 jaws The lower jaw alone is also relatively lighter to the cranium. ^ The frontal and occipital 

 reo-ions are less developed than the parietal. Two points are of especial value,— namely, in the 

 female skull the change of direction from the forehead to the top of the head is more sudden, 

 suo-o-esting a definite aiigle, while in man the passage is imperceptible ; and, secondly m man a 

 wedge-shaped growth above the front of the condyle is more developed, so as to thro>. the face 

 higher up. There is no trouble in recognizing a typical skull of either sex ; but in many cases 

 the decision is difficult, and sometimes impossible. 



Surface Anatomy.^It is convenient for many reasons to settle on what shall 

 be called the normal level of the skull. This should be parallel with the axis of the 

 eye when looking at the horizon. It is expressed by a plane passing through the 

 points above the middle of each external auditory meatus and the lowest points of 

 the anterior border of each orbit. A simple method is to regard the upper border 

 of the zygoma as horizontal, but this is not sufhciently accurate with skulls of low 

 races. The following parts are easily explored by the finger : the whole of the vault 

 as far as the superior occipital line, the occipital protuberance behind, and the supe- 

 rior temporal ridges at the sides. Often the bregma and sometimes the chief sutures 

 can be made out. The possibility of parietal depressions is to be remembered in cases 

 of injury ; also that they may be expected to be symmetrical. 



The superciliary eminences and the upper borders of the orbits are easily 

 explored. The prominence of the former is likely to imply a large frontal sinus ; 

 but the converse is not true, for, especially in the latter part of life, there may be a 

 large sinus with no external indication. The sinus always extends downward to the 

 inner side of the orbit, but its expansion outward and backward is very uncertain. 

 The external angular process protects the outer side of the eye, and one or both 

 temporal ridges can be followed from it. The suture between the process and the 

 malar is easily felt through the skin. A line connecting the most prominent points 

 of the zygomatic arches indicates the depth of the orbits. 



The zygoma is easily followed backward to the auricle. By pressing the latter 

 forward, the supramastoid crest can be made out. Just below this is the spina supra- 

 meatum, close to the cartilaginous meatus. The outside of the mastoid is easily 

 explored. The course of the lateral sinus is in a curved line with the convexity 

 upward from the external occipital protuberance to the upper part of the mastoid, 

 only the lower part of the sinus touching a straight line between those points. 

 According to Birmingham, the descending part follows roughly the line of the 

 attachment of the ear. There is, however, great variation in its course as to the 

 sharpness of its descent and its relation to the surface of the mastoid. It may 

 be exceedingly close, or in no particular relation to it (Figs. 199, 200, and de- 

 scription of the temporal bone, page 179). The antrum leading to the mastoid 

 cells is just back of the upper part of the meatus, often under a small, smooth 

 surface. 



The antrum of Highmore in the superior maxilla extends upward to the floor 

 of the orbit, outward into the malar prominence, downward to'just above the line of 

 reflection of the mucous memi)rane from the lips to the alveolar process, and inward 

 to the line of attachment of the ala of the nose, which is above the canine eminence 

 and marks the separation of the antrum from the nasal cavity. 



The variations of the upper end of the infundibulum are of interest. In the 

 cases (about one-half) in which it drains the frontal sinus it is easy for fluid from the 

 latter to run through the infundibulum both into the nasal cavity through the hiatus 

 semilunaris and into the antrum through the opening in its outer side. If the 



^ Gurriere and Massetti : Rivista speriment. di Frenialria e de Med. legale, 1895. 



