22 PRINCIPLES OF FEEDING FARM ANIMALS 
The white blood-corpuscles (so-called leucocytes) are of larger 
size than the red ones, and are found in only small numbers com- 
pared with red corpuscles. The leucocytes have the power of going 
through the walls of the capillaries (p. 31), and can pass with the 
lymph i in between the cells of the tissues. In case some part of the 
body is injured or diseased, they collect there in large numbers, and 
on breaking down form pus. Their main function appears to be to 
destroy disease germs. 
The muscular tissues in animals consist approximately of 75 per 
cent water, 20 per cent protein, largely myosinogen (myosin), be- 
longing to the globulin group 3 per cent fat, less than 1 per cent 
carbohydrates (glycogen and dextrose), 0.2 per cent nitrogenous 
waste products (so-called extractives), and 1.0 to 1.5 per cent salts. 
The extractives are mainly creatin, with xanthine, uric acid, urea, 
and other waste products that are present in small quantities. The 
mineral matter in the muscle consists largely of potassium phos- 
phates ; small amounts of salts of sodium, calcium, magnesium, and 
iron are also present. 
Connective tissues form another class of nitrogenous organic 
substances in the animal body. To this group belong tendons, liga- 
ments, cartilage, skin, horns, hoofs, etc. They are all insoluble in 
water or salt solutions, and are only slightly attacked by acids or 
alkalies. Among tle substances of this group that have been 
identified only two need be mentioned here, collagen and keratin. 
The former is the main organic component of cartilage and bone, 
and also makes up a large proportion of tendons and ligaments. 
On long boiling with water, collagen is dissolved and forms gelatin, 
which solidities on cooling. Keratin is the main component of 
skin, hoofs, horns, wool, hair, and feathers, all substances that offer 
the greatest resistance to the action of solvents. Keratin contains 
4 to 5 per cent sulfur in addition to the elements ordinarily found 
in protein compounds. On treatment with steam under pressure 
it is rendered soluble and forms glue. The manufacture of this 
material is an important side-line of the large -packing houses. 
Milk contains two important protein substances, casein and 
albumen. Casein belongs to the so-called nucleoproteins, combina- 
tions of albumen and phosphoric acid. It is suspended in a colloidal 
state in milk, and is not in perfect solution, hence may be separated 
out by means of centrifugal force. On addition of an acid to the 
milk, or through the action of exzymee, like rennin or pepsin, 
casein is precipitated, and the milk “curdles.” The manufacture 
of cheese from milk depends on this property of casein. Milk 
