30 THE HORSE IN MOTION. 



If we lay this flake of muscle upon a plate and scrape it gently in the 

 direction of its fibres with a dull knife, we shall find upon the edge of 

 the knife a red pulp without apparent fibre or tenacity, and there will 

 be left behind a bundle of strong cellular tissue. It is to the former 

 that the tractile property belongs ; the latter has no more active power 

 than other cellular tissue ; yet this pulpy bundle of fibres, as muscle, 

 contracts under the stimulus of the will with almost inconceivable 

 power. Borelli estimated that the force exerted by the deltoid muscle 

 of man in supporting a weight held horizontally in the hand was two 

 hundred and nine times greater than the weight. Therefore a weight 

 of sixty pounds held horizontally requires an expenditure of contractile 

 force of the extensor muscles at the shoulder of more than six tons. 

 He demonstrated that the force of the extensors employed by a porter 

 in carrying a weight of one hundred and fifty pounds upon the shoul- 

 der exceeds three tons.* It follows that this enormous power is 

 exerted on the extensors of each leg alternately. 



The natural stimulant to the muscle is the will transmitted through 

 the nerves; but the will is not necessary to muscular contraction, as it 

 has no influence on the muscles of animal life or the vegetative func- 

 tions of animals, and any of the voluntary muscles may be cut off from 

 communication with the brain by severing its nervous connection ; yet 

 contraction may be excited in the muscle so cut off, and this may be 

 continued indefinitely by further division to a microscopic degree ; still 

 the fibres will be observed to contract upon the slightest touch, so 

 closely are the nervous fibres interwoven with those of the muscle. 



Electricity when passed through the muscle in a broken current is 

 a strong excitant to muscular contraction, overmastering the will, and 

 will even cause contraction after life has left it ; but if the current is 

 continuous, it has no such power. 



The muscular fibres are paralyzed by certain poisons, and stimulated 

 to violent contraction by others ; and in disease, as tetanus, they may 

 be so violently stimulated as to be torn asunder. This subject, though 

 very interesting, is leading away from the special inquiry to which we 



* Cyclopaedia of Anatomy and Physiology, art. " Animal Motion." 



