322 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



are also given by which Man is distinguished anatomically from 

 the forms most closely related i.e., the higher apes. 



The proportions of the human body and the structure of many 

 of its parts are influenced by two chief factors, (1) the erect 

 attitude, and (2) the mental development. 



(1) Man is the only Mammal in which the erect position is 

 habitual, easy, and maintained without the help of the upper 

 limbs. In accordance with this the lower limbs, which are much 

 the longer, have become specialized as supports and means of 

 progression, with corresponding loss of mobility in the feet. The 

 relatively short upper limbs, on the contrary, are capable of 

 performing exceedingly complex movements, and the hand is 

 capable of the most delicate manipulation as a combined result 

 of extreme flexibility, the presence of a long thumb opposable to 

 the remaining digits, flat nails, and a delicate sense of touch. 

 These characteristics are associated with corresponding features 

 in the structure of the skeletal and muscular systems. 



(2) Omitting psychological characteristics, although these are 

 by far the most distinctive, the relatively great mental develop- 

 ment of Man is associated with exceedingly large cerebral hemi- 

 spheres, and this again profoundly influences the proportions of 

 the skull. 



Comparison with Rabbit. 



In human anatomy the body is supposed to be in a vertical position, so 

 that upper (superior), lower (inferior), front (anterior), back (posterior) 

 are used as the respective equivalents of the terms front (anterior), back 

 (posterior), lower (ventral), upper (dorsal), as used in Comparative Anatomy. 

 The latter set of terms will be employed here. 



1. Skin. The covering of hair is very much reduced as regards 

 strength and thickness, but the only large areas entirely devoid 

 of hair are the palms of the hands and soles of the feet, this being 

 associated in the former case with very great tactile sensibility. 



2. Endoskeleton. 



I. Axial Skeleton. (1) The skull is accurately poised upon 

 the vertebral column, the two occipital condyles being situated 

 about the middle of its base and the occipital plane being hori- 

 zontal. The cranium is of enormous relative size, in correlation 

 with the dimensions of the brain, while the facial region is 

 reduced, owing to the smaller development of the olfactory 

 capsules, and the reduction of the jaws, which are no longer 



