THE THORAX. 797 



is shown by the fact that its interruption, by mechanical or chemical 

 interference with the respiratory organs, is speedily followed by death. 

 Air-breathing animals are quickly suffocated by strangulation, by im- 

 mersion in water, by placing them under the receiver of an air-pump, 

 and then exhausting it, by giving them only a limited supply of air, 

 or by making them breath gases not containing free oxygen. Aqua- 

 tic breathers are as quickly destroyed, if the fluid by which they are 

 surrounded has been deprived of air by boiling, or by placing it under 

 the receiver of an air pump, and then exhausting it. 



In studying the respiration of Man, and the Mammalia generally, 

 we have to consider the structure of the organs of respiration, i. e. of 

 t{ie thorax and its muscles, the air passages and the lungs ; the mech- 

 anism of respiration, or the respiratory movements by which air is 

 alternately drawn into, and expelled from, the body ; the movement of 

 the air in- respiration, and the capacity of the lungs ; the changes 

 which the air undergoes during respiration ; the changes produced by 

 this process upon the blood and the tissues ; the circumstances which 

 modify the respiratory interchanges, including the phenomena of 

 asphyxia, and the effects of breathing bad air ; and lastly, the organs 

 and functions of respiration in animals. Afterwards, it will be neces- 

 sary to consider the phenomena of Animal Heat, Light, and Electri- 

 city ; and to discuss, in a separate Section, the interesting questions 

 relating to the Dynamics of the Animal Economy. 



THE ORGANS OF RESPIRATION. 



The Thorax. 



The thorax (p. 31, Figs. 10, 13, 14) is an osseo-cartilaginous frame- 

 work filled in with soft tissues, which contains and protects the central 

 organs of respiration and circulation. It corresponds with the dorsal 

 region of the spine. In front, it is formed by the sternum and the 

 cartilages and anterior parts of the ribs ; behind, by the dorsal verte- 

 brae and posterior portions of the ribs ; and at the sides, by the 

 remainder of the ribs. Between these solid parts are the intercostal 

 muscles, which are overlaid in parts by other muscles. 



The cavity of the thorax is conical, being narrow above and broad 

 below. Its upper opening, inclosed between the first dorsal vertebra, 

 the first ribs, and the top of the sternum, is wider transversely than 

 from front to back ; its plane inclines downwards and forwards. The 

 lower opening, bounded by the ensiform cartilage, the last dorsal ver- 

 tebra, and the lower ribs or their cartilages, is much larger than the 

 upper one. It is also wider from side to side than from front to back, 

 but its plane inclines downwards and backwards, so that the thoracic 

 cavity is much deeper behind than in front. 



The upper opening transmits besides certain muscles of the neck, 

 the large bloodvessels of the head and upper limbs, numerous nerves 

 and lymphatics, the thoracic duct, and the oesophagus the principal air- 

 tube, the trachea, or windpipe, which leads to the lungs ; the summits 

 of the two lungs ascend beyond this opening. The lower opening is 



