236 THE CRANIA OF THE SIMIIDAE (PRIMATES) [SECT. A 



To support the enlarged head with its increased weight, the 

 muscles at the back of the neck hypertrophy and extend their 

 areas of attachment about the occipital region. The inion (or point 

 of attachment of the ligamentum nuchae (I.) moves upwards, the 

 whole of the cranial arc or vault is elongated and slightly de- 

 pressed, and the final result on the foramen magnum is that 

 indicated by the position of the point (Op'.). 



3. The last factor to be noticed is an increased growth of 

 the cerebellum. In order to explain this change in an adequate 

 fashion, a much more detailed exposition is needed than can be 

 provided here. Suffice it to say that if the cerebellum becomes 

 enlarged in an antero-posterior direction, it will tend to thrust 

 backwards the lower part of the occipital bone (between I. and Op., 

 Figs. 158 and 159) and will thus cooperate with the factors de- 

 scribed above (as 1 and 2 respectively) in securing the final result. 



In concluding this sketch of a very complicated process, I must 

 add a few remarks by way of comment. 



(a) For the accurate comparison of the various skulls, a first requisite is 

 the employment of a reliable "base-line" to which the successive tracings can 

 be fitted. This need has been realized by the three authorities cited above. 

 And it is remarkable that Professors Bolk and Keith, who have studied 

 principally the skulls of the large anthropoid apes, employ lines which are 

 nearly identical, representing closely the longer diameter of the brain, or to 

 be precise the longest diameter of the cranial cavity. Professor Leche differs 

 in selecting the line of the palate (posteriorly to the anterior palatine foramen); 

 but it is to be remarked that this is very closely parallel to the base-lines 

 adopted by his colleagues, though I am not aware that this coincidence has 

 been recognized. 



(b) The parts of the skull beneath the frontal lobes of the brain undergo 

 comparatively few secondary changes in the later (post-natal) period. Yet 

 I believe that the enormous extension of air-spaces in the brow-ridges of the 

 Gorilla must have an effect in the case of that animal, though the Orang-utan 

 may not be affected thus. Such changes would be supplementary to and in 

 the same direction as those already described. 



(c) Professors Bolk and Keith note that the brain (or the cranial cavity) 

 of the Gorilla is not increased in sectional area after a very early stage in 

 infancy. Indeed, actual diminution of the vaulting is described, and is 

 demonstrable. I find it hard to believe that the arrest or diminution is 

 absolute, and I have evidence to shew that while the height of the cranium 

 is decreased, some compensation takes place by way of an increase in trans- 

 verse width. This is naturally inappreciable when longitudinal sections are 



