IN THE LOWER ANIMALS, ETC. 343 



in the full-grown animal, the external differences 

 in the size of the skulls being almost entirely due 



Fig. 57. — Skull of young Orang-outan ; the capacity of brain-case being 

 nearly equal to that of the adult. 



to the increased thickness of the brain-case, and 

 the very great development of the bony ridges, 

 arising from the long- continued stimulus of the 

 muscles employed in mastication. 



Tig. 58. — Skull of adult Orang-ouian, showing great development of the 

 bony crests for attachment of the muscles. 



The contrast is even more striking in the chim- 

 panzees. The brain in these animals attains nearly 

 its full development at an early period of life, and 

 the growth of the bones, muscles, and the general 

 organic structure proceeds rapidly to completion 



