20'2 PROCEEDINGS OF THE ANATOMICAL AND ANTHROPOLOGICAL 



7. MODIFICATIONS IN THE FOKM OF THE SKULL PRODUCED BY 



MUSCULAR ACTION. 



A considerable variation in the form of the skull is produced by 

 variations in the size of the temporal muscles. In the lower races of 

 mankind the lower jaw is massive, and the temporal muscles are cor- 

 respondingly great in size, and extend upwards over the sides and 

 vault of the skull towards the middle line. In the dry skull the size 

 of the temporal muscles may be estimated by the depth of the tem- 

 poral fossaj, and the height to which the temporal ridges rise upon the 

 parietal bones. In the lower types of man, such as the Australian 

 aborigines, and in certain of the fossil skulls, the ridges are high and 

 near the middle line, indicating that the temporal muscles are rela- 

 tively large. The development of these muscles is especially great in 

 the gorilla, in which the temporal ridges actually meet in the middle 

 line, and are raised up into a median keel or crest, which gives origin 

 by its sides to the more superficial fibres of the temporal muscles, 

 thereby considerably increasing their area of attachment. 



In congenital idiots, the temporal ridges of the skull are also 

 found to be high, and to approach the middle line ; in them, however, 

 the muscles are not actually larger than in healthy individuals, but 

 they are relatively larger when compared with the size of the cranial 

 portion of the skull, which is much diminished. 



The depth of the temporal fossre also depends largely upon the 

 development of the temporal muscles, and the post-orbital constric- 

 tion previously mentioned as characterising the lower types of 

 humanity, is partly, although not wholly, attributable to this 

 cause. 



A similar relation of the muscular ridges on the skull to the size 

 of the muscles which are attached to it, is seen in the occipital region, 

 where the nuchal lines are found to be more pronounced, and extend 

 upwards on the occipital bone farther, in lower types of man, than in 

 higher. 



