ELEMENTARY PHYSIOLOGY. 
lOI 
cells. As examples of the co-ordination between muscle and nerve, 
we have only to observe the rapidity with which an animal, if 
pricked in the leg, lifts that member almost immediately, and again 
there is the well-known saying, his mouth watered at the sight of 
so many .good things," as showing the stimulation of the salivary 
glands by the sight of food. 
Nutrition of the tissues : blood and lymph. — The blood 
carries the necessary nourishment to the tissues and receives their 
waste products. It is however enclosed in a system of vessels 
and does not come into direct relationship with the cells. 
Outside the blood vessels and bathing the cells is the lymph. 
Lymph may be looked upon as the middleman between the blood 
and the tissues ; it carries nourishment from the blood to the tissues, 
and receives in return the waste products of the tissues to be 
carried back to the blood. 
Blood is composed of a clear fluid (plasma) in which is 
suspended millions of small discoid particles — the blood corpuscles 
or blood cells. These blood cells are of two different kinds and are 
generally spoken of as red cells and white celjs. The colour of the 
"blood is due to these red cells. 
When blood is withdrawn from living vessels, it clots ; the red 
cells being the heaviest, sink to the bottom ; the white cells form the 
next layer, while above all little drops of a clear amber-coloured 
fluid make their appearance which, running together, form the 
serum. Clotting is due to changes in the plasma as this fluid 
coagulates in the absence of corpuscles. 
Serum is not plasma in that it does not contain the protein 
fibrinogen, but otherwise it resembles plasma both in appearance and 
composition. It contains various proteins, the most important being 
serum albumen, the amount of which varies according to the condition 
-of the animal. 
The other organic substances in serum may be classified as — 
(i) Substances to be used by the tissues : — 
[d) Glucose, a carbohydrate which is usually in larger 
amount in blood going to muscles than in that 
coming from muscles. This difference is more 
marked when the muscles are active. 
[b) Fats, which occur in varying amounts depending on 
the nature of the food. 
(ii) Substances given off by the tissues, i.e., the waste 
products of tissue metabolism. The principal of these 
is urea which occurs constantly in minute quantities. 
The principal inorganic constituents of serum are the chloride, 
