Chap. XLIII.] ANIMAL MECHANICS. 507 



at a place can be estimated from the same formula. 

 Thus, in a seconds pendulum, where t = 1, 



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7T 2 



CHAPTER XLIIL 



ANIMAL MECHANICS. 



IN the animal body the system of bones connected 

 together by means of joints, and movable on one 

 another by the contraction of muscles, is found to form 

 an arrangement of levers. All the three orders of 

 levers described in chapter xli. are found exemplified 

 in the human body. The fulcrum is offered by the 

 joint, the power is given by the muscular contraction, 

 and the weight is the resistance to be overcome in the 

 movement of the part, the lifting of some weight, etc. 

 Of the first order of levers a good example is afforded 

 in the means by which the head is maintained in the 

 erect position. The fulcrum is the articulation between 

 the condyles of the occipital bone and the atlas, the 

 weight is the weight of the fore part of the head and 

 face, and the power is supplied by the muscles passing 

 upwards to the skull behind, the fulcrum being be- 

 tween the power and weight. The feature of this 

 lever, as one conducing to stability, is seen in the ease 

 with which the head is held up. That it is so held by 

 voluntary muscular effort is evident from the fact 

 that it tends to fall forward, so that the chin rests on 

 the breast, when unconsciousness comes on. When 

 the fore-arm is flexed, and extension is performed 

 by the triceps muscle, we have another example of a 

 lever of the first order, the joint being between 



