io84 LIFE : OUTLINES OF GENERAL BIOLOGY 



that a thin wide band of muscle, that covers the neck and throat in 

 reptiles, spread up over the primitive mammal's face, between the 

 skull and the skin, and formed the muscles that move the lips and 

 the external ears, and that close the eyes. 



But we must not delay any longer in noticing what was going on 

 inside the head — the relative increase of the brain in proportion to 

 size of the whole, and especially the growth of the forebrain, which 

 is the seat of the chief mental processes. In reptiles the brain is 

 almost ridiculously small in proportion to the skull; it comes to its 

 own in birds, and still more in mammals, except the most primitive 

 types. This increase of brains was associated with a raising of the 

 roof of the skuU and the beginning of a forehead. 



la fr 



Fig. 191. 



Bones of a Tiger's Skull, so, supra-occipital; pa, parietal; co, coronoid process 

 of the lower jaw or mandible; fr, frontal; la, lachrymal; mx, maxilla; 

 na, nasal; px, premaxilla; C2, canine tooth of lower jaw; ci, canine tooth 

 of upper jaw; ca, carnassial or sectorial tooth of upper jaw; jti, jugal; 

 a, angle of the mandible; au, ear-hole above the tympanic bulla; pa, 

 paroccipital process of the ex-occipital. Among the salient adaptations of 

 this skull may be mentioned — the posterior crests for muscle-insertion; 

 the prominent zygomatic arch, formed by jugal in front and squamosal 

 behind, for muscle-insertion ;]i within this arch the spacious temporal 

 fossa for jaw-muscles; and theteeth suited for carnivorous habits. 



Another advance must be associated with the arboreal life of 

 certain pioneering mammals, beginning with forms like the Tree- 

 Shrews, the Spectral Tarsier and the Lemurs, and going on to 

 monkeys and apes. The emancipation of the hand from being a 

 support was associated with the reduction of the snout. An epoch- 

 making increase in that part of the cerebrum that is concerned with 

 vision, attention, delicate manipulation, and the like, was correlated 

 with a forehead worthy of the name. The eyes were directed for- 

 wards, as snout decreased and forehead increased; the human face 

 was within sight! There were, of course, other factors, such as the 

 teeth, and the food they dealt with, the evolving voice and the role 

 of lips and tongue in articulation, the moulding influence of varia- 

 tions in the hormones that regulate growth, the need for expressing 



