Because the muscles which form their mechanism are voluntary muscles—that is, they are subject to the will of man, and influenced by impulses directed to them through the nervous system by the mind, which is the governing power.
"And he took him by the right hand, and lifted him up; and immediately his feet and ankle bones received strength."—Acts iii.
948. Why does the heart beat without any effort of the will?
Because the muscles of the heart are involuntary muscles—that is, they are independent of the will, and receive a continuous nervous stimulus which is not under the controul of the mind.
Fig. 58.—MUSCLES AND VESSELS OF THE LEG AND FOOT.
A. A large ligament, which covers the knee pan, or moveable bone of the knee, by which the ends of the bones of the thigh and leg are kept from slipping over each other.