64 



ANIMAL MECHANISM. 



every case it is an estimate of the specific force of a certain 

 volume of steam which, is to be determined. 



In the same manner, in hydraulic machines, we must know 

 the charge of water or its pressure, in order to ascertain the 

 work which the machine can perform. 



Physiologists have also sought to determine the specific 

 force of muscular tissue in difierent animals, and to compare 

 with the unit of transverse section of muscle the effort which 

 it can make. In this manner they have estimated that the 

 muscle of the frog would develop an effort of 692 grammes 

 (E. Weber) for each square centimetre of section ; that human 

 muscle would develop 1087 (Koster) . In the bird the force 

 would be about 1200 (Marey) ; in the insect it would, be still 

 greater (Plateau). 



According to Straus Durkheim, a muscle of the stag-beetle 

 weighing 20 centigrammes would carry, if we measure the 

 moment of power and that of resistance a weight of seven 

 kilogrammes. 



By such estimates as these, we might compare animated 

 moving powers with machines working under variable pres- 

 sures. The frog, we might say, works with a pressure less 

 than one atmosphere, man with a pressure greater than one 

 atmosphere. There would be a greater pressure in the bird, 

 and still greater in the insect. 



Of machines, — When mechanical force cannot be directly 

 utilized, because it is not in harmony with the form of work 

 which it ought to effect, various means are employed in the 

 arts to transform it. Machinery known under the names of 

 wheels and levers are continually used for this purpose. In 

 the animal organism contrivances are also found which change 

 the form of the work of the muscles. The lever is almost 

 exclusively used by nature for this purpose. The arrange- 

 ment of the bony levers which form the skeleton is so generally 

 known that it needs no explanation here ; but there is a very 

 common error on this point, even among physiologists, which 

 it is necessary to point out. 



Almost all the levers which are found in the organism belong 

 to the third order, that is to say, where the muscular force is 

 applied between the fulcrum and the resistance. Under these 



