6 
MAMMALIA. 
organs suspended to the two sides of tlie chest — and of the tube 
called the trachea, which puts the lungs in communication with 
the external air. The trachea is a cylindrical membranous tube, 
at first single, and which then separates into two parts, called the 
bronchial tubes , which soon lose themselves in an infinite number 
of little ramifications in the midst of the substance of the lung. 
The ramifications of the bronchial tubes may be compared, in 
their form, to the roots of a tree. The lining of the rami- 
fications of the bronchial tubes is formed of a membrane of a 
loose texture, permeable to the air, and which allows it to pass 
freely into all the cells of the pulmonary tissue. It is in this 
tissue that the capillary vessels, which are to extend as far as the 
pulmonary veins, come and abut ; and it is thus that the venous 
blood finds itself exposed to the action of the oxygen, which 
modifies its nature and transforms it into arterial blood. 
The mechanism of respiration is effected by the elevation of the 
ribs and the contraction of the diaphragm. 
The diaphragm is a flat- muscle, which separates the cavity of 
the abdomen from that of the chest. It is fixed, on one side, to 
the vertebral column, and on the other, to the base of the bony 
framework formed by the sternum and the ribs. When it con- 
tracts, it diminishes the transversal diameter of the chest, by 
increasing its antero-posterior diameter ; then, and by the effect 
of the atmospheric pressure, the air precipitates itself into the 
lungs by the mouth or by the nostrils, and by following the course 
of the bronchial tubes, penetrates into all the pulmonary cells. 
Such is the phenomenon of inspiration. Then the diaphragm 
becomes relaxed, the ribs and the pulmonary cells, by their own 
elasticity, return to their original positions, and drive out the 
gas with which they were filled. This phenomenon is called 
expiration. During the sojourn of the air in the ramifications of 
the lung, the oxygen of the air inspired is combined with the 
elements of the blood ; in such a manner that the composition of the 
gas which issues from the lungs is very different from that of the 
air inspired. The gas driven out of the lung during the expiration 
contains less oxygen, and is loaded with a considerable quantity 
of carbonic acid gas, the oxygen of the air combining with the 
rejected carbon which is conveyed by the venous blood into the lungs. 
The respiratory movements vary much in their frequency 
