454 RESPIRATION AND EXCRETION 
arteries in waves which can be felt in the wrist or neck by 
placing the finger over an artery. The wave is called the 
pulse. By counting the number of waves each minute, the 
rate at which the heart beats is determined. When a person 
runs or takes violent exercise, the pulse rate increases. It 
is advisable to know what our 
usual pulse rate is, for an in¬ 
creased pulse rate is sometimes 
an indication of approaching 
illness. 
Closely associated with the 
pulse is the additional condition 
known as blood pressure. Blood 
pressure is the force with which 
the blood pushes against the 
walls of the arteries with every 
heart beat. Special appliances 
have been devised which accu¬ 
rately measure the amount of this 
pressure. The information thus 
revealed to the skillful physician 
is often very important. 
Lymph. — As the blood flows through the capillaries, 
part of the plasma passes through the thin walls into the 
spaces between the cells and bathes the cells. This fluid 
which escapes from the capillaries is called lymph (limf). 
It is composed of digested food, water, and other substances. 
The cells assimilate the food which they need and cast back 
into the lymph the wastes which they have formed in the 
process of growth and repair. These spaces between the cells 
are small and irregular in shape. The spaces, however, form 
a sort of mesh, or net, the parts of which join, forming larger 
vessels, and finally all the lymph is collected into two large 
vessels which open into veins. Thus there is the lymphatic 
circulation which differs from that of the blood in several 
To Kidneys^-'i. 
/To Large Intestine- 
..-To Leg 
Figure 404 . — Main Arteries 
of Frog. 
