40 PHYSIOLOGY CHAP. 



Lactic acid has been proved experimentally to be one of the 

 normal katabolites of muscle, formed not only in dead but also 

 in living muscle during rest, and still more during work. On 

 artificially circulating defibrinated blood for three hours through 

 the muscles of the lower limbs of a dog, the amount of lactic acid 

 that can be extracted from the blood that has repeatedly passed 

 through the resting muscle amounts to about T5 grins. Tetanisa- 

 tion of living muscle certainly increases lactic acid formation ; 

 the amount of lactates present in the blood (Spiro) or excreted by 

 the kidneys (Colasanti and Moscatelli) increases. The muscles 

 do not, however, acquire an acid reaction, because the lactic acid 

 is given off as fast as it is formed to the blood - stream, where 

 it is saturated with alkali. When, on the contrary, a group of 

 muscles previously cut off from the circulation is tetanised they 

 become acid owing to accumulation of lactic acid, while the 

 corresponding, non-excited muscles of the opposite side remain 

 neutral or alkaline and contain little lactate (Marcuse, Werther). 

 In excised frog's muscle slight electrical excitation, which causes 

 no violent contraction, suffices to convert the neutral into an acid 

 reaction (Gotschlich). 



From these and other experimental researches it may be con- 

 cluded that the formation of lactic acid is associated with the life 

 of muscle, and not with its death, as many believe. The con- 

 vincing evidence of this lies in the fact that when a muscle with 

 normal circulation is tetanised, then excised, it forms less acid 

 during its death than the corresponding muscle which was not 

 excited, showing that the mother-substance of the acid has been 

 used up, and that the amount of acid developed by a muscle in 

 dying corresponds with the quantity of mother-substance con- 

 tained in it. 



In addition to protein and glycogen the fats may be regarded 

 as reserve materials; these are found not only in the inter- 

 muscular connective tissue, but also within the fibres and in the 

 sarcoplasm, and especially in the fibres of the red muscles, in the 

 form of droplets which give them a turbid appearance (Ph. Knoll). 

 Some of these droplets stain black with osmic acid, others remain 

 unstained and probably consist of lecithin. During starvation 

 they disappear, and return on feeding. In morbid degenerative 

 changes, as after phosphorus poisoning, the amount of fat in- 

 creases enormously, and it must therefore be due not to storage, 

 but to regressive metamorphosis of the proteins. 



The part played by the fats in muscular metabolism is un- 

 known. The small fat-content of normal fibres is no reason for 

 regarding it as unimportant, since in all probability fat does not 

 accumulate normally because it is consumed as soon as formed. 

 According to Bogdanow the fat of muscle-substance is richer in 

 volatile fatty acids than that of the inter muscular connective 



