23G 
The Applications of Physiology 
own practice, witlioiit entering into your domain of practically 
applying the theories ; which I am sure you yourselves can do 
much better than I can, and with less risk of error. I therefore 
recommend to your own consideration whether impro\ ements in 
practice may not be made from the principles now developed. 
But, before leaving the consideration of food, it may be ad- 
visable to explain to you the effects which common salt produces 
on the animal economy. You are all aware of the practice of giving 
salt to cattle, and of its beneficial effects. 1 have mentioned 
repeatedly that respiration is carried on by means of the com- 
bustion of those constituents of the food which are destitute of 
nitrogen. But before this combustion is effected, they are trans- 
formed by the liver into the fluid called t)ile. Bile is a compound 
of the alkali soda, with a resinous or highly carbonaceous sub- 
stance derived from the food. The bile, after being formed, is 
absorbed by certain vessels of the intestines, and there meeting 
with oxygen, is consumed and converted into carbonic acid and 
water, which are expired by the lungs. The combustion does 
not take place in the lungs themselves, as is generally supposed, 
but in the intestines. For this theory we are also indebted to the 
great German chemist. The manner is very curious in which 
the carbonic acid is carried up and expired by the lungs. All 
blood contains iron. TJiere are two principal states of oxidation 
of this metal — viz. the peroxide and the protoxide. The former 
yields oxygen very readily to organic matter, and occasions its 
conversion into carbonic acid and water ; but in doing so, it is 
reduced to the state of protoxide, which oxide has a great affinity 
for the carbonic acid thus formed : it unites w ith it, and is car- 
ried up by the venous blood to the lungs as carbonate of iron. 
There it comes in contact with the oxygen of the air. But the 
moment carbonate of iron is acted upon by oxygen it is decom- 
posed, peroxide of iron being again formed ; whilst the carbonic 
acid is set at liberty, and is expired by the lungs. The peroxide 
of iron in the arterial blood executes its function anew, as carrier 
of carbonic acid to the lungs. 
Now as bile is the medium through which respiration is sup- 
ported, it is necessary that it should be properly and regularly 
formed. This can only be done by supplying the animal with a 
constant amount of soda : this we do in common salt. The soda 
of the salt aids in the formation of the bile, whilst its muriatic 
acid assists the digestion of the food. A proper formation of the 
secretions is necessary to the health of an animal, and a supply of 
salt is highly favourable to their production. 
But, whilst it is admitted that a limited supply of salt is very 
useful to the health of the animal, a large supply is highly pre- 
judicial, and prevents the formation of fat. An experiment was 
