Dec. 15, 1924 
Composition o f Diet, Blood , and Milk 
621 
siderable change in the protein. As a 
result there was a drop in the concen¬ 
tration of amino N in the blood and 
plasma, a drop in milk yield, and a 
drop in the concentration of milk N 
(figs. 1, 2, and 7). When the energy 
of the ration was restored to the initial 
level, the amino N of the blood and 
plasma, the milk yield, and the con¬ 
centration of milk N all rose (figs. 
4 and 7). The amino N in the blood 
and plasma was determined so long 
after the second change in ration that 
its rise has not been shown in any of 
the figures, but it may be seen in 
Table IV. The results of this experi¬ 
ment may be taken as particularly 
strong and consistent evidence for the 
view that the free amino acids of the 
blood play a predominant part in 
regulating milk secretion. This is 
rather surprising when we bear in mind 
that only the energy content of the 
ration in this experiment was changed. 
In Experiments III and IV the 
quantity of dietary protein was reduced 
without change in its quality and with¬ 
out change in the dietary energy; and 
in Experiment III the protein was after 
a time restored to the initial level. In 
Experiment I the dietary protein and 
energy were reduced together and later 
restored to the initial level. In all 
three of these experiments the milk 
yield and the concentration of milk N 
followed the changes in dietary protein 
in the same sense in which they fol¬ 
lowed the dietary energy in Experi¬ 
ment II (figs. 3, 4, 6, and 8). In 
all three cases, also, Jhere is reason to 
think that the reduction in dietary 
protein was followed by a reduction 
In the level of the plasma amino N 
(fig. 1). These results are therefore 
like those of Experiment II, consistent 
with the view that the dietary changes 
affected milk yield through the free 
amino acids of the blood. 
In all three of these experiments, 
however, the plasma amino N began 
to rise while the cows were still on the 
inadequate rations and while the milk 
yields in two of the cases were still 
falling off (figs. 1 and 3). Further, 
when the rations were restored to the 
initial levels in Experiments I and 
III, there was a marked tendency for 
the milk yields and the concentration 
of milk N to rise in both cases, though 
in neither case was there any tendency 
for the plasma amino N to rise. 
The results of Experiments V and VI 
how even more clearly that there is 
no hard and fast relation between the 
rate of milk secretion and the con¬ 
centration of total amino N in the 
blood plasma. In these experiments 
both the quantity and the quality of 
the dietary protein were changed. 
When the quantity and quality of the 
dietary protein were reduced together, 
there was a decided tendency for the 
milk yield to fall off and for the concen¬ 
tration of milk N to be reduced; and 
both these tendencies were reversed 
when the quantity and quality of the 
dietary protein were increased (figs. 
3, 4, 9, and 10). But reduction in the 
quantity and quality of the dietary 
protein caused no significant change in 
the level of plasma amino N (fig. 1); 
and the opposite changes in the dietary 
protein were followed in both experi¬ 
ments by decided reductions in the 
level of plasma amino N (fig. 2) in 
spite of the rising milk yields and 
rising concentrations of milk N. 
The lack of parallelism between 
plasma amino N and milk yield, which 
is to some extent apparent in all the 
Fig. 9—Experiment V. Changes in the composi¬ 
tion of milk. Concentration in period 1 is taken 
as 100 per cent. In period 2 the protein of the 
ration was reduced in both quantity and quality; 
in period 3 it was restored to adequacy 
experiments, but particularly in ex¬ 
periments V and VI, is no real reason 
for abandoning the hypothesis that in 
these experiments the milk yield was 
controlled through the plasma amino- 
acid mixture. The manner in which 
milk yield may be increased by a 
change in the plasma amino-acid 
mixture, even though the concentra¬ 
tion of total plasma amino N may be 
at the same time reduced, is shown by 
the results of Experiments IV, VI, and 
VIII, in which the plasma amino N 
and the free tryptophane of the blood 
were followed simultaneously. (Tables 
VI, VIII, and X.) These experiments 
have already been discussed to some 
extent. They show that when the 
quantity of protein in the ration is 
reduced, the composition of the blood 
mixture of amino acids may be very 
considerably altered; that when the 
milk yield falls off rapidly during a 
period on a ration inadequate in 
protein, and the plasma amino N at 
the same time gradually rises, the 
concentration of tryptophane in the 
blood may constantly fall; that when 
