538 THE MECHANICAL FUNCTIONS. 



posture can be permanently maintained, and in 

 which the office of supporting the trunk of the 

 body is consigned exclusively to the lower extre- 

 mities. To this intention the form and arrange- 

 ment of all the parts of the osseous fabric, and 

 the position and adjustments of the organs of 

 sense have a well marked reference.* The 

 lower limbs are qualified to be the efficient in- 

 struments of progression by their greater length 

 and muscularity, compared with the generality 

 of quadrupeds. The only exceptions to this rule 

 occur in those mammalia which are constructed 

 expressly for leaping, such as the Kanguroo and 

 Jerboa, where, however, the hind legs are em- 

 ployed almost solely for that mode of progression. 

 The Quadrumana, which come nearer to the 

 human form than any of the other tribes, have the 

 lower limbs comparatively weak. In almost all 

 other quadrupeds the disproportion is still greater, 

 the thigh being short, and almost concealed by 

 the muscles of the trunk, and the remainder of 

 the limb being slender, and not surrounded by 

 any considerable mass of muscles. 



* In most quadrupeds, as we have seen, the thorax is deep in 

 the direction from the sternum to the spine, but is compressed 

 laterally, for the evident purpose of bringing the fore limbs 

 nearer to each other, that they might more effectually support 

 the anterior part of the trunk. In Man, on the contrary, the tho- 

 rax is flattened anteriorly, and extends more in width than in 

 depth ; thus throwing out the shoulders, and allowing an exten- 

 sive range of motion to the arms. 



