ELEMENTARY PHYSIOLOGY. 
are the abdominal muscles which compress the viscera, pushing them 
upwards and thus press the diaphragm further up into the chest. 
At the same time they pull back the ribs and thus decrease the size 
of the chest from side to side. 
Special respiratory movements, i.e., coughing, sighing and 
yawning — 
(^) Coughing is due to irritation of the respiratory tract and 
is produced by a forced inspiration followed by a strong 
effort at expiration, during which the glottis (opening of 
larynx) is closed but is forced open by the current of 
expired air. 
(b) Sighing and yawning are deep involuntary inspirations. 
They accelerate the circulation of the blood when from 
any reason this becomes less active. 
(2) The interchange of gases between the air in the air cells 
and the blood. — This is effected partly by diffusion and partly by 
the activity of the air cells lining the vesicles of the lung. Carbonic 
acid gas and water are exchanged for oxygen, which is in turn 
exchanged for carbonic acid, etc., from the tissues. 
Excretion of matter from the body. — -We have already 
seen that the waste and useless material in the food, together 
with a certain quantity of water, is got rid of by the bowels. 
We have also seen that a certain amount of the products of 
tissue metabolism is excreted by the lungs ; but there is still a 
certain amount of waste material which has not been accounted for, 
i.e., the remainder of the water and the waste nitrogen of the 
body ; these are chiefly eliminated by the urine which is secreted by 
the kidneys. The kidneys may be roughly described as a collection 
of cells arranged more or less in th# form of little tubules betw^een 
which capillaries run. Each tubule commences as a small sac 
surrounding a tuft of vessels ; these little tubules collect and run 
together until at last we find in the centre of the kidney a small 
fan-shaped cavity which leads into the duct of the kidney — the 
ureter. As the blood passes along through the kidney, the cells of 
the little saccules and tubules extract from it a certain amount of 
water and waste solids, the most important being urea, which is the 
form in which about 80 per cent, of the waste nitrogen leaves the 
body. As it is secreted, or rather excreted, the urine passes down 
the ureters to the bladder, and when a certain amount has collected, 
the bladder contracts and it is discharged. 
