926 SPECIAL PHYSIOLOGY. 



ceous fuel, or a machine for converting potential energy into mechanical force, 

 and that it does not undergo much more waste when in action, than when at 

 comparative rest. Besides this, he believes that the oxidizable material does 

 not require to be previously organized or made part of the muscle, but only to 

 be digested and assimilated into the blood, of which it forms a part. He con- 

 ceives that the materials of the food, together with oxygen, circulate in the 

 blood going through the muscle, and that when the latter is quiescent, no 

 chemical action takes place ; but that when a muscle is excited to act by a 

 nerve, the nerve force determines the oxidation of non-nitrogenous matters in 

 the blood, and so sets free potential energy, partly acting as heat, and partly 

 as motion. Dr. Frankland admits, however, that nitrogenous matters may 

 also be employed for this purpose, as is illustrated by the work performed by 

 men and animals fed on flesh diet. But ordinary flesh contains much fatty 

 matter. 



Dr. Parkes has observed the effects of exercise and rest, under different diets, 

 on the excretion of urea, over longer periods than those noticed by Fick and 

 Wislicenus. His results likewise show, that on a non-nitrogenous diet, exer- 

 cise produces no increase in the excretion of nitrogen ; that less urea is excreted 

 during the period of actual work ; and that, the elimination of nitrogen, is regu- 

 lated,"rather by the character of the diet than by the amount of exercise. The 

 subjects of observation were two healthy soldiers, whose normal daily standard 

 of excretion of nitrogen, was first ascertained during a period of six days, in 

 which they took their ordinary food and exercise. For two days they con- 

 .sumed non-nitrogenous food, and rested ; the urea, and the total nitrogen ex- 

 creted, then fell to a mean or less than one-half the normal quantity, and yet 

 .the men lost weight. They next returned for four days, to their ordinary diet 

 and occupation ; the nitrogen excreted, as urea and otherwise, immediately in- 

 creased from day to day, but did not, on the last day, reach its normal standard ; 

 and the total quantity excreted in the four days was less than half of that 

 eliminated in four of the first six days ;. some nitrogenous food was apparently 

 retained for the nutrition of the tissues, or to supply the nitrogenous blood- 

 .material expended in the two days of non-nitrogenous diet. For the next two 

 days, the men again took a non-nitrogenous food, but instead of resting, they un- 

 derwent full exercise, walking, on the first day, 23.76, and on the second, 32.78 

 miles ; the food satisfied the sense of hunger which was felt ; much fatigue was 

 experienced, especially on the second day ; the excretion of nitrogen decreased 

 during the first thirty-six hours, but in the succeeding twelve hours, which 

 were hours of rest, it showed a marked increase; the pulmonary and cutaneous 

 excretions increased, the former 100, and the latter 50 per cent. ; the men lost 

 weight. Finally, being allowed their usual diet, with ordinary exercise, the 

 quantity of urea again rose daily, and at last surpassed the normal quantity. 

 The chief difference in these results, as compared with those of Fick and Wis- 

 licenus, whose observations were not sufficiently prolonged, is in the increased 

 excretion of nitrogen, during the hours of rest, after severe exercise on a non- 

 nitrogenous diet. This may merely show that the effects of the changes taking 

 place in the muscles during exercise, are slow to manifest themselves in the 

 excreta. The diminution in the nitrogenous excretion, during actual work, 

 on a non-nitrogenous diet, may, as Parkes suggests, be owing to nitrogen 

 being then retained and used, and not to the entire absence of decomposition 

 in the muscular tissues. 



In subsequent experiments on this subject, Dr. Parkes found that upon an 

 ordinary mixed diet, containing a daily quantity of about 19.6 grammes of 

 nitrogen, rather less of that element was excreted during the early periods of 

 exercise, and during actual exercise, than during rest, especially during the 

 rest immediately after work,- when the quantity rose, so as to be excessive. 

 He suggests a new explanation of the facts, viz., that a muscle increases in 

 size when in action, then appropriating more nitrogen than it loses ; but that 

 when it is at rest, it lessens in bulk, losing more nitrogen than it appropriates. 

 Muscular movement is regarded as due to a process of formation, and repose 

 as accompanied by disintegration. The non-nitrogenous substances surround- 

 ing the ultimate muscular elements, undergo change during the action of the 

 muscle ; the effete products, chiefly of those non-nitrogenous substances, as 



