THE PROPERTIES OF MUSCULAR TISSUE. 135 



Then if we hang one gram (15.5 grains) on it and give it the 

 same stimulus, it will be found to contract more, say four or 

 five millimeters, and so on, up to the point when it carries 

 eight or ten grams. After that an increased weight will, 

 with the same stimulus, cause a less contraction. So that up 

 to a certain limit, resistance to the shortening of the muscle 

 makes it more able to shorten : the mere greater extension of 

 the muscle due to the greater resistance opposed to its short- 

 ening, puts it into 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 

 contraction, 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 

 foikeii by a contraction, and increases its height. Heat rigor 

 is produced in excised frog's muscle by heating it to about 

 40 0. (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 occur- 

 rence of muscular rigor. 



The Measure of Muscular Work. The work done by a 

 muscle in a given contraction, when it lifts a weight verti- 

 cally against gravity, is measured by the weight m'oved, mul- 

 .tiplied 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 compensates for the less distance through which it is 



