458 



MOTION. 



second only backwards, and the third neither 

 backwards nor forwards. 



The length of the legs, measured from the 

 hip-joint to the ground when standing erect, 

 preponderates slightly over that of the body 

 when taken from the distance of the crown of 

 the head to the axis of the hip joint, conse- 

 quently the centre of gravity is raised above the 

 plane of position rather higher than the semi- 

 distance from the head to the ground when the 

 entire sole of the foot is in contact with the 

 earth.* In consequence of the lengths of the 

 femur and tibia being nearly equal, and of the 

 zigzag direction which the limbs take during 

 flexion, and because the angle of the greatest 

 flexion of the knee-joint is nearly equal to the 

 sum of the angles of the hip and ankle, it 

 results that in the simultaneous flexion of 

 these three joints, (provided the angle of in- 

 clination of the foot to the ground continues 

 30,) the trunk preserves its erect position as 

 when standing, and therefore ascends and de- 

 scends (by the flexion and extension of the legs) 

 in the same vertical line. The greatest amount 

 of elongation and contraction of the leg result- 

 ing from the greatest extension and flexion of 

 the hip, knee, and ankle-joints, may be easily 

 ascertained by measuring with a line from the 

 hip-joint to the head of the astragalus during 

 such flexion and extension, and marking the 

 difference in the length of the line in the two 

 states. By this method the length is estimated 

 by Weber as follows : 



m.m. 



In the greatest state of extension. . 924.26 

 In the greatest state of flexion .... 404.74 



The actual lengths of the several portions of 

 the leg are 



m.m. 



Head of the femur to the condyles . . 380.0 



From the condyles to the convex 



articular surface of astragalus . . 420.0 



Axis of articulation of the astragalus 



to centre of foot 136.0 



During the greatest extension the first three 

 of these points of the leg formed an angle of 

 1488', and during the greatest flexion an angle 

 of 40. The second, third, and fourth points 

 form in the greatest extension an angle of 

 157 4, and in greatest flexion 94 6'. 



The proportion between the greatest and 

 smallest length which the leg can assume varies 

 but little. 



Figs. 247 and 248 from Weber show the 

 greatest amount of extension and flexion which 

 the leg can effect; the corresponding difference 

 in length is as 14 to 5, but in walking and run- 

 ning the flexion is less, and the difference of 

 length as 11 to 9. 



In standing, the weight of the body is trans- 

 mitted from the pelvis to the heads of the 

 femurs ; the oblique directions of the latter 

 upwards and inwards keep in equilibrio the 

 vertical forces downwards and outwards pro- 

 duced by the weight of the body on the wedge- 

 like sacrum. 



The femurs transmit the weight impressed 



* Sec positions of the centre of gravity, sect. i. 

 p. 409. 



Fig. 247. 



Fig. 348. 



on them by the trunk, together with their own 

 weight and that of the soft parts to the tibiae. 

 The" shafts of the latter are straight, and by their 

 prismatic form their solid contents are removed 

 further from the axis of the bone, which enables 

 them to support a greater weight with less ex- 

 penditure of materials. 



The fibula adds likewise to the strength of 

 the tibia, both vertically and laterally ; the 

 latter transmits, in its turn, its own weight and 

 that of its soft parts to the astragalus. 



The astragalus transmits the pressure made 

 upon it partly to the calcaneum and partly to 

 the scaphoides; the calcaneum partly to the 

 ground and partly to the cuboides; the sca- 

 phoides transfers the force through the cunei- 

 form bones ; the toes to the ground, where 

 the base of support terminates, supposing 

 the entire foot to rest on it. In the mechanism 

 of the foot we discover an elastic arch, upon 

 which the shock of the body is received, and 

 transmitted obliquely to the ground. This admi- 

 rable structure prevents the jar which the body 

 would otherwise sustain at each step by the re- 

 action of the ground in walking, running, leap- 

 ing, or falling ; but an exact investigation of 

 the mechanism of the foot would occupy more 

 space than we have assigned to this subject. 

 The areas of the soles of the two feet and the 

 space lying between them is the whole base of 

 support in standing on both legs; but when 

 standing upon one only, this is diminished, 

 not only by the area of one foot, but also of the 

 space lying between them. In the latter case, 

 the base is so narrow that the act is not easily ac- 

 complished, but the difficulty of keeping the 

 trunk in equilibrium i? vastly increased when 

 the base of support is reduced to the area of the 

 great toe, as is accomplished by opera dancers; 

 or to the area of one-fifth or two- fifths of a 

 square inch as in skating, or to one still less, as 

 in rope dancing, wherein the base of support 

 oscillates laterally. In this latter case, the diffi- 

 culty is increased by the necessity of keeping the 

 trunk in equilibrio, but the centre of gravity is 

 retained in the plane of the rope by fixing the 

 eye on some distant point in it. The posi- 

 tion which the two feet ought to take, when 

 equally advanced and equally inclined, so that 



