446 ANIMAL PHYSIOLOGY. 
and better milk than another, and the bull is himself able to 
transmit this peculiarity ; for, when crossed with other breeds, 
he improves the milking qualities of the latter. Individual 
differences are also well known. 
THE METABOLIC PROCESSES CONCERNED IN THE FORMATION 
oF Urea, Uric Acip, HIPPURIC ACID, AND ALLIED 
Bopigs. 
Creatin is represented by the formula C,H,N;0,, and crea- 
tinin by C,H,N;O—that is, the latter may be regarded as the 
firmer dehydrated. Creatinin occurs, as we have seen, in urine, 
and the question arises, Is the creatin of muscle the antecedent 
of the creatinin of urine? Creatin when injected into the 
blood reappears as creatinin in the urine; but the latter sub- 
stance is not increased by exercise, though the creatin of the 
muscles is, while, like urea, creatin is augmented by a proteid 
(flesh) diet. It is not clear, then, that the creatin of muscle 
has any definite relation to the creatinin of urine. But crea- 
tin occurs not only in muscle, but in a variety of other tis- 
sues, including the nervous; in fact, it may be regarded as 
one of the products of proteid metabolism. Putting these 
facts along with the absence of urea itself from muscle and 
many other tissues, there is some probability in the view 
that creatin is one of the antecedents of urea; possibly it is 
one of the products which the kidneys directly convert into 
urea. 
There are several facts which point to the liver as being 
the seat of urea formation: 1. Leucin, when taken in large 
quantities, reappears in the urine as urea, or, at all events, is 
followed by an increase in the excretion of urea by the kid- 
neys. 2. In certain diseases of the liver (acute atrophy) urea 
is largely replaced in the urine by leucin and tyrosin. Now, 
since the consumption of much proteid matter is soon fol- 
lowed by an excess of urea in the urine, and since in such 
cases it is likely that a good deal of leucin and its compan- 
ion, tyrosin, are formed in the digestive tract, which we may 
suppose are carried directly by the portal blood to the liver, 
the conclusion has been drawn from this and the facts just 
mentioned, as well as others, that the liver is a former of 
urea, 
