THE BONES 



69 



are connected with one another by means of well-defined 

 anastomoses. 



Thorax. 



The barrel-like form of the human thorax is due to the 

 erect position (see Fig. 24). The thorax was originally keel- 

 shaped and is so still in all those mammals who support the 

 weight of the body on all four extremities (pachyderms, 

 beasts of prey, ruminants, equidae), the barrel-shaped form 

 being a later development in man and in all mammals the 

 weight of whose bodies is supported by water, by air, or by 

 the posterior extremities 

 (marsupials, rodents, in- 

 sectivora, cetaceans, ot- 

 ters, sea-otters, bats and 

 anthropoids). That the 

 barrel form has been 

 evolved from the keel 

 form can be proved, onto- 

 genetically and phylo- 

 genetically. The deter- 

 mining factor in the 

 transformation of the 

 thorax in the Primates 

 was the gradual conver- 

 sion of the anterior ex- 

 tremities into organs of 

 prehension, with their great muscular development. This form 

 of the thorax causes the centre of gravity to be transferred to 

 the back, thus facilitating the adoption of the erect position. 1 



Man in common with the orang has normally twelve ribs, 

 the gorilla and chimpanzee have thirteen, the hylobates fourteen. 

 As Wiedersheim * states so emphatically, the history of evolu- 

 tion has shown that man's progenitors must have had a greater 

 number of ribs, and it is a fact that a thirteenth rib is present 

 in the human embiyo ; it disappears later on, though in 

 exceptional cases it continues to develop. Traces of super- 

 numerary ribs have been found in the region of the lumbar 



1 Wiedersheim, loc. cit., pp. 39-41. "Ibid., p. 42. 



FIG. 24. The Human Thorax. 

 B, sternum ; Sb, scapula. 



