THE RESPIRATORY SYSTEM. 
Respiration is the process whereby the tissues are sup- 
plied with oxygen and relieved of their carbon dioxid. In 
mammals, the special organs of respiration are the lungs, 
wherein the carbon dioxid is received from the blood, while 
at the same time the oxygen of the air passes through the 
thin-walled capillaries to the red blood-corpuscles capable 
of conveying it to the cells throughout the body. Each 
cell is composed largely of carbon, hydrogen, oxygen, and 
nitrogen, and any activity on the part of the cell is the 
result of the chemical union of some of its elements, 
whereby several waste products are formed, one of which 
is the gas, CO, (carbon dioxid). This gas is a poison and 
therefore must be eliminated. It passes through the thin 
walls of the capillaries adjacent to every cell, and is trans- 
ferred through the veins to the heart and thence to the 
lungs. Here the pulmonary artery divides up into capil- 
laries ramifying over the air sacs (Fig. 84), thus per- 
mitting the carbon dioxid to escape into the air sacs. Other 
waste products resulting from chemical activity within the 
cells are carried away by the kidneys and-sweat glands. 
The respiratory system consists of the nasal passages, 
pharynx, larynx, trachea, and lungs. The air taken in at 
the anterior nares is warmed in passing over the mucous 
membrane of the turbinated bones, after which it goes on 
through the posterior nares (Fig. 18) to the pharynx, and 
thence into the larynx. 
The larynx is the cartilaginous expansion of the cranial 
end of the trachea, at the base of the tongue. The basihyal 
bone is attached to the cranial ventral margin of the larynx 
and on each side is a thyrohyal bone (Fig. 20). Dorsal 
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