344 



Journal of Agriculture. 



[8 June, 1907. 



so economical as some of the latest forms of oil engines which ha^-e an 

 efficiency of close on 40 per cent. 



From the foregoing facts we can now state that, when a muscle con- 

 tracts, it uses up nitrogen-free carbon compounds and oxygen, gives off 

 water and carbonic acid, and produces heat. 



Factors that influence muscular contraction. — -Even though a 

 muscle mav not be contracting visibly we shall find that in a conscious 



Fi^. 41. — The muscle on the right is the gastrocnemius, that on the left is the 

 tibialis anticus. (After Hagemann.). 



animal it is shorter than if the animal were narcotised or asleep. This 

 slight, but constant, contraction, which keeps the muscle taut and readv 

 at a moment's notice to contract forcibly in response to a stronger nerve 

 message, is called ' ' muscle tone. '" ' 



The greater the resistance to be overcome the greater is the force of 

 contraction. This can be seen even in a muscle which has been cut out 

 of a recently killed animal ; within certain limits it liberates more energv 

 and contracts more forcibly, the more it is hampered in its contraction. 

 Then again we are aware that by our will we can make a muscle act 

 strongly or weakly as we are inclined. That is to sav, the force of a 

 muscular contraction will vary according to the nerve message the muscle 

 receives. 



