440 



MUSCULAR MOTION. 



D 



Upper Portion of Thigh 

 Bone. 



Fig. 192. But another and important advantage is that of 



affording additional strength in adventitious cir- 

 cumstances. When we are standing perfectly 

 erect, the necks of the thigh-bones are very oblique, 

 compared with the line of direction of the body; 

 but if we are thrown forcibly to one side, the line 

 of direction of gravitation corresponds more nearly 

 with that of the neck of the thigh-bone, and frac- 

 ture is rarely produced in this manner. The most 

 common cause of fracture of the neck of the thigh- 

 bone is slipping from a curbstone, or any slight 

 elevation, with one foot, upon a firm substance 

 beneath ; and the fracture in such case, is general- 

 ly transverse. The advantage of this arrangement 

 of the neck of the thigh-bone has been compared 

 not inaptly to that resulting from the dishing of a 

 wheel ; or the oblique position of the spokes from 

 the nave outward to the felly, which strengthens 

 the wheel against the strains produced by its sink- 

 ing with force into a rut or other hollow. 1 The 

 femur transmits the weight of the body to the 

 large bone of the leg the tibia ; but, from the mode in which the pel- 

 vis presses upon it, its lower extremity has a tendency to bear forwards. 

 This is prevented by the action of the extensors of the leg rectus and 

 triceps cruris whose power is augmented by the presence of the patella 

 a sesamoid bone, seated behind their tendon. The muscles of the 

 posterior part of the leg, which are attached to the condyles of the 

 thigh-bone, aid also in preserving the equilibrium. 



The tibia is- the sole agent for the transmission of the superincumbent 

 weight to the foot. Its upper extremity has, however, a tendency to 

 bear forwards like the lower part of the os femoris. This is prevented 

 by the contraction of the gastrocnemii, tibialis posticus, and other mus- 

 cles on the posterior part of the leg. 



The foot sustains the whole weight of the body ; and its shape and 

 structure are well adapted for the purpose. The sole has some extent, 

 which contributes to the firmness of the erect attitude. The skin and 

 epidermis are thick ; and beneath the skin is a thick, adipous stratum, in 

 greater quantity at the parts of the foot which come in contact with 

 the soil. This fat forms a kind of elastic cushion, adapted for deaden- 

 ing or diminishing the effect of pressure. The whole of the sole of the 

 foot does not come in contact with the ground. The weight is trans- 

 mitted by the heel, the outer margin, the part corresponding to the 

 anterior extremity of the metatarsal bones, and the extremities or pulps 

 of the toes. The tibia transmits the weight to the astragalus ; and 

 from this bone it is distributed to the others that compose the foot ; 

 but the heel conveys the largest share. When the foot rests upon a 

 flat surface, it is entirely passive ; but when upon a slippery soil, the 

 flexors of the toes, especially of the great toe, are firmly contracted, so 



1 See Fig. 170; al 

 Lond., 1829. 



Sir C. Bell, Animal Mechanics, p. 21, Library of Useful Knowledge, 



