224 PRINCIPLES OF ANIMAL NUTRITION. 
same thing is held to be true of protein so far as it contributes 
energy for muscular exertion. 
As we have seen in Chapter II, however, the formation of dex- 
trose from fat in the liver is by no means universally admitted, and 
Chauveau’s ingenious theory as to the immediate source of muscu- 
lar energy has not lacked opponents. If it is true, fat has a much 
lower value for that purpose than corresponds to its potential 
energy as measured by its heat of combustion. If it be assumed 
to be converted into dextrose in accordance with the equation on 
p. 38, it is easy to compute that about 36 per cent. of its potential 
energy will be liberated as heat in the process and that consequently 
only the 64 per cent. remaining in the resulting dextrose will 
be available to the muscles. Consequently the relative values of 
fat and dextrose for the production of work will be as 162 to 100 
and not as 253 to 100. 
While the evidence of the respiratory quotient is not incon- 
sistent with Chauveau’s theory, it is also not inconsistent with the 
view which supposes fat to be directly metabolized for the produc- 
tion of mechanical work. The difference lies, not in the amounts 
of carbon dioxide and oxygen evolved but in the place where and 
the form in which the energy is liberated, and the question can 
therefore be satisfactorily discussed only on the side of its energy 
relations. 
Postponing that discussion for the present, it may be remarked 
here that while it appears to be true, as already stated, that the 
muscular glycogen and the dextrose of the blood are a source of 
muscular energy, and perhaps the most readily available one, it 
by no means follows that they are the only source. The muscle 
contains other non-nitrogenous reserve materials besides glycogen, 
and notably a not inconsiderable amount of fat and of lecithin. 
Moreover, recent investigations (see pp. 63 to 65) have shown that 
the amount of the muscular fat is greater than was formerly sup- 
posed, and that some of it cannot be extracted with ether and 
behaves almost as if in chemical combination. Indeed, it appears 
not improbable that both fat and carbohydrate molecular groupings, 
as well as proteids, enter into the structure of living protoplasm. 
Finally, not only the muscle but the blood which nourishes it 
contains fat as well as carbohyhrates, the former indeed being more 
