6 MAMMALIA. 



organs suspended to the two sides of the chest — and of the tube 

 called the trachea, which puts the lungs in communication with 

 the external air. The trachea is a cylindi-ical membranous tube, 

 at first single, and which then separates into two parts, called the 

 bronchial tubes, which soon lose themselyes in an infinite number 

 of little ramifications in the midst of the substance of the Ixmg. 

 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 

 pulmonai'}' 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 eifected by the elevation of the 

 ribs and the contraction of the diaphragm. 



The diaphragm is a fiat muscle, which separates the cavitj' 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, peueti-ates into all the pidnionary cells. 

 Such is the i^henomenou 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 

 I'.rpiration. During the sojourn of the air in the ramifications of 

 t:ie 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 Imigs 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 combinino- with the 

 rejected carbon which is conveyed bj'the venous blood into the lun<TS. 



The respiratory movements vary much in their frequency 



