THE MUSCULAR SYSTEM 409 



work the oxygen used up and carbon dioxide produced are both 

 increased, and the increase provides some measure of the work 

 performed. Even during rest a muscle is doing work, for we 

 have learnt that it is always in a condition of tonus — viz., of slight 

 contraction. Since a muscular contraction is essentially the 

 outcome of an oxidational process, the storage of oxygen by the 

 resting muscle may be said, in Pfliiger's words, ' to wind it up ' 

 in preparation for its contracting activity. This accounts for the 

 fact that an excised muscle of the cold-blooded frog can be made 

 to give some hundred contractions when suitably treated. In 

 mammalian muscle, on the other hand, with its much greater 

 metabolic activity, this quiescent storage of oxygen does not 

 suffice to maintain its irritability for more than the briefest 

 interval' after its blood supply is cut off. 



In an active muscle the bloodvessels are more dilated than in 

 the muscle at rest, and this dilatation provides for the increased 

 quantity of blood now required by the part. By means of the 

 blood the irritability of the muscle, or its power of contraction, 

 is maintained ; whatever leads to a smaller quantity of blood 

 being sent to an active muscle produces partial or complete 

 paralysis of the group or groups of muscles affected. This is 

 well seen in the horse when suffering from thrombosis of the iliac 

 arteries ; the blood supply is sufficient during the time the animal 

 is at rest, or even at a walk, but if called upon to trot muscular 

 cramps occur, followed by paralysis. 



The study of metabolism has prepared us for the statement 

 that the chemical changes occurring during contraction do not 

 normally affect the nitrogenous elements of the muscle. There 

 is probably no increased output of nitrogenous substances such 

 as kreatin. The excretion of any increased amount of urea is 

 variable, irregular, may even be non-existent, and is in no case 

 remotely proportional to the work done. This is true as long as 

 the body is supplied with a sufficiency of the non-nitrogenous 

 carbohydrates and fats. If they are deficient, then increased 

 muscular activity does lead to an increased formation of urea, 

 since the muscle now has to metabolise its proteins to provide 

 the energy necessary for the work performed and the heat simul- 

 taneously produced. The main products of muscular waste are 

 therefore to be looked for in the destruction of stored-up carbo- 

 hydrate material. 



Carbohydrates and fats are normally the chief source of 

 muscular contraction. The carbohydrates are represented by 

 glycogen, formed, as we have seen, by the sugar brought to the 

 muscle, and by dextrose. A well-nourished muscle contains 

 0-5 to 0-9 per cent, of its weight as glycogen. This local store 

 of animal starch is, under the influence of an enzyme, converted 



