DEGREE OF IRRITABILITY OF MUSCLES AND NERVES. Ill 



resistance. As the load continues to be increased, the height of the con- 

 traction 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 by multiplying the number of units 

 of height to which the load is raised into the number of units of 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, as may be seen from the follow- 

 ing example : 



Load, in grammes .... 50 100 150 200 250 

 Height of contractions, in millimetres 14 9 7 5 2 



Work done, in gram-millimetres . 450 700 750 400 



80. The influence of 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 will be the shortening caused 

 by the same contraction wave ; the greater will be the height of the con- 

 traction 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 larger number of fibres that is to say, the fibres being 

 of equal width, which has the greatest 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, 

 the work done will be the greater in the one which has the larger bulk, 

 which contains the greater number of cubic units. In speaking^ therefore, 

 of the work which can be done by a muscle, we may use as a standard a 

 cubic unit of bulk ; or, the specific gravity of the muscle being the same, a 

 unit of weight. 



We learn, then, from the foregoing paragraphs that the work done by a 

 muscle-nerve preparation will depend, not only on the activity of the nerve 

 and muscle as determined by their own irritability, but also on the character 

 and mode of application of the stimulus ; on the kind of contraction 

 (whether a single spasm, or a slowly repeated or a rapidly repeated tetanus) 

 on the load itself, and on the size and form of the muscle. Taking the most 

 favorable circumstances viz., a well-nourished, lively preparation, a maxi- 

 mum stimulus causing a rapid tetanus, and an appropriate load we may 

 determine the maximum work done by a given weight of muscle, say one 

 gramme. This in the case of the muscles of the frog has been estimated at 

 about four gram-metres for one gramme of muscle. 



THE CIRCUMSTANCES WHICH DETERMINE THE DEGREE OF IRRITABILITY 

 OF MUSCLES AND NERVES. 



81. A muscle-nerve preparation at the time that it is removed from 

 the body possesses a certain degree of irritability ; it responds by a contrac- 

 tion of a certain amount to a stimulus of a certain strength applied to the 

 nerve or to the muscle. After a while, the exact period depending on a 



