528 PHYSIOLOGY AT THE FARM. 



on with great activity, it will probably be soon brought to an 

 end. To enter into it to any extent would lead us too far from 

 our proper objects. We decidedly incline to Professor Playfair's 

 view, both as respects the correspondence between the quantity 

 of urea excreted in a given time and the amount of muscular 

 effort performed in that time, and also as respects the restric- 

 tion of the non-nitrogenised aliment to the office of producing 

 sensible animal heat. 



When a fascicle (a fasciculus or bundle of muscular fibres) 

 contracts between two bones connected by a flexible joint, the 

 bone more movable at the moment approaches to that which 

 is more fixed. This is a purely mechanical act, produced by 

 the shortened state of the muscle. Here no conversion of heat 

 into mechanical effect is requisite, but the shortening of the 

 fascicle is the result of a movement among the organic ele- 

 ments of each of the contained fibres. This is plainly the 

 movement into which heat is converted. It is known that the 

 presence of oxygen is essential to this movement; and what is 

 more probable than that the heat generated by its presence, 

 through its effect in combining with the carbon of the tissue, 

 is the sole source of the heat available for the mechanical effort ? 

 Any heat generated in the mean time by non-azotised princi- 

 ples is wholly external to the agents in operation, and could 

 aid in the effect no more than a red-hot hammer would in- 

 crease the force of the muscles with which the blacksmith 

 deals his blows to the anvil. If the urea formed in a given 

 time do not always correspond to the amount of muscular 

 exertion, as when it is very violent, it is far more probable that 

 other nitrogenised products are then generated such as am- 

 monia than that heat, arising from the combustion of non- 

 nitrogenised principles, is converted into mechanical force. 



A few words of recapitulation, in reference to Professor Play- 

 fair's conclusions, will appropriately wind up the views hitherto 



