LOCOMOTION ON SOLIDS. THE ERECT POSTURE. 171 



have presented a condition of constant insecurity. The great relative 

 strength, and length, of the human femur, are also associated with the 

 firmness of posture, and the rate of locomotion, of man. 



At the hip-joint, the depth of the acetabulum, especially the over- 

 hanging of its upper border, the increased protection afforded by its 

 fibrous rim, the presence of the internal ligament (ligamentum teres), 

 and of the strong accessory ligament over the front of the joint, give 

 great security to this part of the frame ; whilst the slightly arched 

 form of the femur, the accumulation of compact bony tissue at the 

 back of this bone, and, we may alo add, the prismatic shape of the 

 tibia, and the brace-like provision afforded by the presence of a com- 

 plete fibula, are evidences of special adaptation in the supporting 

 osseous columns of the lower limb. 



The strong hoop-like mass of bone formed by the pelvis, is adapted 

 for the steady support of the superincumbent weight of the rest of the 

 trunk, and for the transmission of this, downwards to the thighs ; its 

 circular form imparts to it great strength ; its inclined position from 

 before backwards, serves to sustain the viscera within it ; and the 

 greater thickness of its bony tissue along the lines extending on each 

 side outwards and downwards from the sacrum to the upper part of 

 the acetabula, insures sufficient strength in the directions through 

 which the weight is transmitted from the lower part of the vertebral 

 column to the two hip-joints, and thence to the heads of the thigh 

 bones. The large and projecting surfaces of the pelvis are occupied 

 entirely by muscular attachments, and they afford great leverage for 

 those muscles which pass upwards from the lower limbs, and serve to 

 balance the pelvis upon the thighs. 



The great size of the upper part of the sacrum, and its mode of at- 

 tachment between the innominate bones, the breadth and mass of the 

 bodies of the lumbar vertebrae, the gradually diminishing size of these, 

 and of the dorsal and cervical vertebrae, in passing from below up- 

 wards, in accordance with the successively diminished weight which 

 they have to bear, are arrangements evidently in harmony with the 

 erect position of the body (see Figs. 10, 11, 12). The length and 

 breadth of the spinous processes correspond, severally, with the 

 strength of the muscles connected with them at different parts of the 

 spine, serving to increase their leverage and their surfaces of attach- 

 ment. Moreover, the direction of these, as well as of the transverse 

 processes, is horizontal in the loins and neck, so as to permit of rota- 

 tory movements in those regions ; whilst, in the back, the spinous 

 processes overlap each other, and the transverse processes are so con- 

 nected with the ribs, as to impede such rotation, whereas they do not 

 prevent extreme forward bending in that part of the vertebral column. 

 In the neck and loins, the bending of the column takes place chiefly 

 in the backward direction. The presence of the intervertebral sub- 

 stances, Figs. 10, 12, and their effect in imparting elasticity and di- 

 minishing shock, have already been noticed. Lastly, the curvature? 

 of the spinal column itself, presenting an anterior convexity in the 

 neck, a posterior convexity in the back, and, again, an anterior convexity 

 in the lumbar region, also increase the elasticity of the vertebral column, 



