CHAP. V] THE CRANIA OF THE SIMIIDAE (PRIMATES) 237 



studied, and I should be prepared to find my suggestion confirmed when a 

 correspondingly detailed study of coronal or horizontal sections has been 

 undertaken. At present, however, the amount of information on this point 

 is comparatively scanty. 



(d) Reference has been made to the influence of the nuchal muscles on 

 the form of the cranial envelope. This aspect of the question has been 

 studied by Professor Leche in greater detail than by the other observers. 

 A very important point is involved, viz. how far the muscles are capable of 

 influencing the cranial form. The authorities are divided on this question. 

 I have formed the opinion that Professor Leche is right in ascribing an im- 

 portant part to the nuchal muscles, though (as I understand his account) he 

 demurs to the view whereby an equally potent force is granted to the tem- 

 poral muscles. In this place it must suffice to indicate the diversity of views, 

 and to repeat that the changes under discussion are encountered in many 

 mammals, and that the factors are numerous and variable in intensity. All 

 this is admirably stated by Professor Leche. 



(e) Mention has been made [cf. paragraph (b) suprci] of the frontal sinuses 

 and the jaws. An account of the development of these would need a separate 

 section. The subject is important here on account of its intimate relation to 

 the processes of cranial development, i.e. the evolution of the brain-case 

 proper. Again with advantage reference may be made to the publication of 

 Professor Leche. The changes attain in some ways a maximum of intensity 

 in the animal form (Mycetes) studied by him ; and the Howling Monkey is 

 therefore of much use in enabling us to recognize the slighter degrees of 

 change met with in other animals. In this part of the subject, the question 

 of the air-spaces in the facial bones needs even further study than has been 

 accorded to it by Professors Bolk and Keith ; and in particular, the remark- 

 able extension of air-spaces into the palatine roof of the Chimpanzee (as 

 recorded by Professor Bolk) should be specially noted. 



(/) A word of warning remains to be given in regard to the "fixity" of 

 the central part of the cranial base. This part is included between the points 

 B. and Pr. in the various illustrations (viz. Figs. 150 et seq.). The fixity or 

 constancy of this part (B. Pr.) is not absolute, for even this central portion 

 participates in the changes consequent on the increased size of the brain. 

 For this reason, this line is not entirely satisfactory as a " base-line,", and 

 although it was enq>loyed in that way by Huxley, we have seen [paragraph 

 (a) supra] that it has been superseded by other lines in recent investigations. 

 Yet for the general realization of the processes attendant on the evolution of 

 the skull in Man and the Simiidae, this line still possesses advantages so 

 great and the lack of fixity is relatively so small that I decided to retain it 

 in the first instance. 



From the foregoing paragraphs we may learn how the study 

 of the skull in various types of animal (whether mammals or 

 other) gives guidance as to their cerebral or encephalic con- 



