THE PROPERTIES OF MUSCULAR TISSUE 95 



a state in which it is able to contract more powerfully. Fatigue 

 diminishes the working power of a muscle and rest restores it, 

 especially if the circulation of the blood be going on in it at the 

 same time. A frog's muscle cut out of the body will, however, be 

 considerably restored during a period of rest, even although no 

 blood flow through it. 



Cold increases the time occupied by a simple muscular contrac- 

 tion, and also impairs the contractile power, as we find in our own 

 limbs when " numbed " with cold, though in that case the hurtful 

 influence of the cold on the nerves no doubt also plays a part. 

 Moderate warmth on the other hand, up to near the point at which 

 death stiffening (often in this case spoken of as heat rigor) occurs, 

 diminishes the time taken by a contraction, and increases its 

 height. Heat rigor is produced in excised frog's muscle by heating 

 it to about 40 C. (104 F.) The required temperature is higher 

 in warm-blooded animals, especially while the circulation through 

 the muscle is maintained: in fevers temperatures considerably 

 greater than the above have been observed without the occurrence 

 of muscular rigor. v 



The Measure of Muscular Work. The work done by a muscle in 

 a given contraction, when it lifts a weight vertically against grav- 

 ity, is measured by the weight moved, multiplied by the distance 

 through which it is moved. When a muscle contracts carrying no 

 load it does very little work, lifting only its own weight; when 

 loaded with one gram and lifting it five millimeters it does five 

 gram-millimeters of work, just as an engineer would say an engine 

 had done so many kilogrammeters or foot-pounds. If loaded with 

 ten grams and lifting it six millimeters it would do sixty gram- 

 millimeters of work. Even after the weight becomes so great that 

 it is lifted through a less distance, the work done by the muscle 

 goes on increasing, for the heavier weight lifted more than com- 

 pensates for the less distance through which it is raised. For ex- 

 ample, if the above muscle were loaded with fifty grams it would 

 maybe lift that weight only 1.5 millimeters, but it would then do 

 75 gram-millimeters of work, which is more than when it lifted 

 ten grams six millimeters. A load is, however, at last reached 

 with which the muscle does less work, the lift becoming very little 

 indeed, until at last the weight becomes so great that the muscle 

 cannot lift it at all and so does no work when stimulated. Starting 



