444 MUSCULAR MOTION. 



mitted, and this is a cause why the spine admits of the greatest mo- 

 tion anteriorly. In rotation, the whole is pressed upon and undergoes 

 elongation in the direction of its constituent laminae. In old age, the 

 cartilages become shrivelled; and this, with the loss of muscular power, 

 is one of the causes why old people bend forwards. 



When we assume different positions with the trunk, the centre of 

 motion of the vertebrae becomes modified. If we bend forwards, it is 

 thrown on the anterior part of the body of the vertebrae; if to one side, 

 on the articulating processes, &c. Each vertebra, we have seen, is a 

 lever of the first kind; and as the centre of motion becomes altered 

 the leverage must be so likewise. It is when the body has been bent 

 forwards, and the object is to restore it to the erect position, that the 

 power acts with the greatest advantage, the fulcrum being thrown to 

 the anterior part of the body of the vertebra, and the arm of the power 

 being the distance between this point and the extremity of the* spinous 

 process into which the power is inserted. 



Each vertebra has but a slight degree of motion ; but the sum of all 

 their motions is considerable, and is estimated by .multiplying the 

 single motion by the number of vertebrae. The result, however, can 

 only be regarded as approximate, as the extent of motion, of which the 

 different vertebrae are capable, necessarily varies. The arrangement 

 of the spinous processes of the vertebrae especially of the dorsal 

 prevents any considerable flexion of the body backwards: and when 

 we find the tumbler bending his body back until his head touches his 

 heels, it is owing to the arrangement of the spine having been modified 

 in early life by constant efforts of the kind, until there are no longer 

 obstacles to the movement. 



The motions of the vertebrae are frequently united to those of the 

 pelvis on the thigh-bones, so that they seem to be more extensive than 

 they really are. This is the case when we make a low bow. 



The motions of the spine are inservient to those of the head, and of 

 the superior and inferior extremities. 



The upper limbs are capable of various motions; some of which have 

 been already described ; others will be hereafter. They are useful in 

 the different attitudes ; and, at times, by transmitting to the soil a part 

 of the weight of the body, and thus enlarging the base of sustentation, 

 as when we employ a stick, rest on the hands and knees, or support 

 the head on one or both elbows. They are of great use, likewise, 

 in preserving equilibrium when we walk on a very narrow base ; serv- 

 ing in part the purpose of the pole employed by the dancer on the 

 tight-rope. 



The lower extremities are, of course, locomotive organs; but they 

 are susceptible of partial movements likewise; as when we kick with 

 one foot, try the consistence of the ground, cross the legs, tread the 

 foot-board of a lathe, &c. 



Thus much for the attitudes. We shall now consider the mode in 

 which the relation of the body to the soil is altered, comprising the 

 physiology of walking, leaping, running, swimming, flying, &c., which 

 constitute the different varieties of locomotion or progression. 



