128 LECTURE XII. ' . 



animal, which is employed in drawing a distant boat or carriage, by the in- 

 clination of the rope or chain to the horizon, compared with the weight of 

 that portion of it which the animal supports, that is, of the part which extend* 

 to the point where the curve becomes horizontal. 



All animal actions, or, at least, all the external actions of animals, are ulti- 

 mately dependent on the contractions and relaxations of the flesliy parts, - 

 which are called muscles. The operation of the particular muscles belongs 

 properly to the Iscience of physiology ; but their mechanism may in general 

 be understood from the properties of the lever and of the centre of gravity. 

 ^\\t bones are the levers, the joints the fulcrums, and the force is applied by 

 the muscles, which are usually attached to the bones by the intervention of 

 tendinous cords. When a muscle contracts in the direction of its fibres, it 

 becomes at the same time thicker, and its total bulk is little if at all diminish- 

 ed: when it relaxes itself, it is merely passive, for the fibres, being extremely 

 flexible, can have little or no effect in separating the parts to which they are 

 attached; this separation is generally performed by the action of other mus- 

 cles, which are called the antagonists of the first, but sometimes by clastic 

 ligaments, or by other means. The bone forms a lever of the second kind, 

 where the two forces opposing each other are on the same side of the fulcrum. 

 In general the insertion of a muscle is much nearer to the fulcrum than the 

 point of action, and the obliquity of its direction gives it a still greater me- 

 chanical disadvantage with regard to rotatory power; but it is more conve- 

 nient in the animal economy to produce a great contractile force, than 'a great 

 extent in the original motion. For instance, when the arm is raised by the 

 exertion of the deltoid muscle of the shoulder, a very strong contraction takes 

 place in the muscle, but the action is only continued through a short ^pace ; 

 had the contractile power been weaker and more extensive, the shoulder must 

 have been made higher, in order to give it sufficient purchase, and the pro- 

 jection would have been inconvenient. 



Borelli has calculated that the immediate force of the biceps, or double- 

 headed muscle which bends the arm, is equivalent to about 300 pounds, and 

 that of the muscles which raise the lower jaw, above 500 in man, but in beasts 

 of prey far greater. It is obvious that in muscles of the same kind, the 

 strength must be as the number of fibres, or as the extent of the surface which 



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