410 A MANUAL OF VETERINARY PHYSIOLOGY 



back to sugar, and in this form is capable of being converted into 

 energy under the influence of oxidising enzymes ; as we have 

 previously seen, the contraction of muscle leads to the disappear- 

 ance of glycogen. In this condition, though free from glycogen, 

 the muscle still continues to contract, and if the view be adopted 

 that sugar is the source of muscular energy, there is still sufficient 

 for the purpose existing in the blood, which even under starvation 

 continues to maintain its normal o-i per cent. Under starvation 

 this sugar is furnished by the breaking down of body protein, 

 but under the ordinary condition of nutrition, when work has 

 emptied the muscles of glycogen, there is ample sugar available 

 from the store of carbohydrate taken into the body. Many 

 circumstances seem to point to sugar as being necessary to the 

 chemical changes occurring during contraction. Quantitative 

 investigations on the muscular work performed by man, as 

 measured by the ergograph, has, it is said, shown that sugar 

 increases the amount of muscular work which can be performed, 

 yet it has never been proved that sugar is an essential food for 

 muscular activity. Unfortunately, the suggestion of its utility 

 has, in the hands of ignorant laymen, frequently led to deplorable 

 errors in the dieting of working-horses. 



The changes occurring in muscles as the results of contraction 

 may be tabulated as follows : 



i. More oxygen is consumed. 



2. More carbon dioxide is produced. 



3. Glycogen is used up. 



4. Lactic acid is formed. 



5. Heat is produced. 



The first three we have already studied. The formation of acid 

 in muscle as the result of contraction has given rise to a good 

 deal of inquiry, and very different results have been obtained. 

 Lactic acid is found in muscle ; it is not, as we have seen, the 

 ordinary lactic acid found in milk, but a form optically active 

 to which the name ' sarcolactic acid ' has been given. It appears 

 clear that resting muscle only contains a trace of lactic acid 

 (0-03 per cent.), while under the influence of contraction a marked 

 amount is present (0*22 per cent.), and rigor, whether pro- 

 duced artificially by heating the muscle or naturally after death, 

 leads to the production of as much as 0-3 to 0-5 per cent. The 

 source of the acid is not agreed upon by physiologists ; some 

 regard it as a product of protein metabolism, others as an inter- 

 mediate product formed from sugar. 



During muscular activity heat is produced ; the blood returning 

 from a muscle has a higher temperature than that going to it. 

 Colin found the temperature of the masseter muscle of the horse 



