502 A MANUAL OF PHYSIOLOGY 



only two-thirds into heat (Zuntz). In hard work three- 

 quarters of the energy may be changed into heat ; but even 

 then the efficiency of the muscles far outstrips that of the 

 best steam-engines, which convert only an eighth of the total 

 energy into work. 



Notwithstanding the splendid efficiency of the muscular 

 machine, the gaseous metabolism easily rises during muscular 

 work to five times, and in severe labour to nine times its 

 resting value, although persons inured to toil work more 

 economically than amateurs. It is, indeed, not difficult to 

 show that the greater part of the metabolism and heat- 

 production of a man doing ordinary work is accounted for 

 by the contraction of the voluntary and involuntary muscles. 



It follows from Hirn's mean results that a 70 kilo man doing 

 27,700 kilogramme-metres of work in an hour gives off 283,000 

 small calories of heat. Now, 27,700 kilogram me-metres = say, 65,000 

 small calories ; and on the assumption that the skeletal muscles 

 produce four or even three times as much heat as work, the con- 

 traction of these alone, without reckoning the heat produced by the 

 heart, would account for by far the greatest part of the heat-pro- 

 duction during the hours of work. Taking the working day at eight 

 hours, at least two-thirds of the total heat-production would be thus 

 accounted for, particularly as a certain additional amount of muscular 

 energy is expended in maintaining the position of the body necessary 

 for the performance of the work. But even in muscles completely at 

 rest metabolism goes on, and some heat is produced. The muscles 

 of a dog's legs, through which an artificial circulation of defibrinated 

 blood is kept up, consume at body temperature on the average about 

 150 c.c. of oxygen per kilo per hour. This is more than one-third of 

 the rate of consumption per kilo of a resting dog, reckoned on the 

 total weight of the animal. For Zuntz found that a fasting dog of 

 26 kilos, lying at rest, consumed oxygen at the rate of 10,260 c.c. 

 per hour, or about 395 c.c. per kilo her hour. Taking the muscles 

 as 40 per cent, of the body-weight, and assuming that oxygen 

 consumption and heat-production are under the given conditions 



approximately proportional, we get ^-^ x -IS = say i : 6 or i : 7 as 



395 I0 



the ratio of the heat-production of the muscles when absolutely at 

 rest, and removed from the influence of the nervous system, to the 

 total heat-production of the resting animal. 



It is probable that in the skeletal muscles of curarized animals the 

 heat-production is not far different from that in isolated muscles at 

 body temperature, and subjected to a good artificial circulation. 

 Now, curara has been found to reduce the oxygen consumption of a 

 rabbit from 770 c.c. to 500 c.c. per kilo per hour. On the assumption 

 that the same proportion holds for a dog, we should get a reduction 



