640 MAN xxiv. 8- 



of bone above the eyes, probably produced to meet the compression 

 stresses set up by the powerful action of the jaw-muscles. Their 

 absence, together with the large forehead, produces the human type 

 of face. The large external nose is presumably another corollary of 

 the shortened face; it provides some extension of the nasal cavity, 

 necessary for warming and filtering the air. 



The balancing of the head on the neck is a result of the adoption of 

 the upright position. Movement of the foramen magnum to a position 

 beneath the skull has been noted as a primate characteristic and it 

 reaches its extreme in man, allowing considerable reduction of the 

 musculature at the back of the neck; the splenius and semispinalis 

 capitis muscles are much smaller in man than in apes. The small size 

 of the trapezius is partly a consequence of the good balance of the 

 head, partly of the absence of brachiating habits. Reduction of these 

 muscles leads to simplification of the bones at both ends of them. The 

 area of their attachment to the occipital surface of the skull becomes 

 much reduced and remains smooth, instead of being roughened and 

 even raised into ridges as in apes. At the same time the spines of the 

 cervical vertebrae, very long in the gorilla, are short and almost 

 vestigial in man. When the head is properly balanced on the backbone 

 it can be freely turned around, and for this purpose the sternomastoid 

 muscles are well developed and the large mastoid ('breast-like') 

 swellings where they are attached to the base of the skull provide a 

 characteristic human feature. 



9. Rate of development of man 



One of the most striking differences between man and apes is the 

 slow rate of our own growth and development; there is a strong 

 suspicion that many of our features are due to retardation of the time 

 of onset of maturity. Schultz has shown that in the apes growth ceases 

 between the ages of 10 and 12 and that the epiphyses finally close 

 between 12 and 14. Many of the features of man, such as the reduction 

 of hair and the large head, presence of a prepuce on the penis and 

 hymen in the vagina, are those to be found in foetal apes, and it is 

 therefore suggested that one of the main changes leading to our 

 development has been delay in the rate of differentiation and onset of 

 maturity. This might well depend on the endocrine balance, perhaps 

 particularly on the action of the anterior lobe of the pituitary. It is 

 only possible to guess at the process of habit change and selection by 

 which the appropriate genetic change has occurred. It may well be 

 that those family organizations were more efficient in which individuals 



