88 THE MUSCLE-NERVE MACHINE. [BOOK i. 



as great, or even greater, than in the first. That is to say, the 

 resistance offered to the contraction actually augments the con- 

 traction, the tension of the muscular fibre increases the facility 

 with which the explosive changes resulting in a contraction take 

 place. And it has been observed by Heidenhain that the degree 

 of acid reaction, the amount of carbonic acid given off and the 

 rise of temperature are greater in a muscle contracting against 

 resistance than when the resistance is removed ; that is to say, 

 the tension increases the metabolism. There is, of course, a 

 limit to this favourable action of the resistance. As the load con- 

 tinues to be increased, the height of the contraction is diminished, 

 and at last a point is reached at which the muscle is unable (even 

 when the stimulus chosen is the strongest possible) to lift the load 

 at all. 



In a muscle viewed as a machine we have to deal not merely 

 with the height of the contraction, that is with the amount of 

 shortening, but with the work done. And this is measured as the 

 height to which the load is raised multiplied into the weight of 

 the load. Hence it is obvious from the foregoing observations 

 that the work done must be largely dependent on the weight 

 itself. Thus there is a certain weight of load, with which in any 

 given muscle, stimulated by a given stimulus, the most work will 

 be done. 



Since mere tension affects the changes going on in the muscular 

 fibres, it is desirable in experiments in which muscles are loaded, that 

 the weight should not bear upon the lever until the contraction actually 

 begins. This is easily managed by interposing between the end of the 

 muscle and the weight a lever with a support so arranged that, before 

 contraction takes place, the weight only extends the muscle to the 

 length natural to it during rest; but that the muscle directly it shortens 

 at once begins to pull on the weight. The muscle is then said to 

 be after-loaded 1 . 



The value of a muscle as a machine is further determined by the 

 Size and Form of the Muscle. Since all known muscular fibres are 

 much shorter than the wave-length of a contraction, it is obvious 

 that the longer the fibre, the greater the height of the contraction 

 with the same stimulus. Hence in a muscle of parallel fibres, the 

 height to which the load is raised as the result of a given stimulus 

 applied to its nerve, will depend on the length of the fibres, while 

 the maximum weight of load capable of being lifted will depend 

 .on the number of the fibres, since the load is distributed among 

 them. Of two muscles therefore of equal length (and of the same 

 quality) the most work will be done by that which has the greater 

 sectional area; and of two muscles with equal sectional areas, 

 the most work will be done by that which is the longer. If 

 the two muscles are unequal both in length and sectional area, 



1 This is perhaps the best equivalent of the German uberlastet. 



