88 OF RESPIRATION. 



rally and interiorly, so that the bodies oi' the six ribs 

 mentioned above (143) are elevated and their inferior 

 margin drawn somewhat outwards, and the arch of 

 the diaphragm at. the same time rather depressed and 

 flattened. 



I have never observed the inferior extremity of the 

 sternum, in tranquil respiration, to be thrust forwards, 

 as some have asserted. (C) 



146. This alternate motion of the chest continues, 

 during health and freedom from restraint, from the 

 hour of birth till death. Its object is, that the lungs 

 may be expanded to admit the air and contracted to 

 expel it, in perpetual alternation. This alternation 

 occurs, in an adult at rest, about 14 times in a minute, 



once to about five pulsations of the heart. 



147. For man, in common with all warrn-blooded 

 animals, cannot long retain the inspired air, but is 

 compelled to discharge it and take in a fresh supply 

 of this pabulum of life, as it always has been denomi- 

 nated.* Common observation teaches, that however 

 pure may be the air entering the lungs, it instantly 

 undergoes remarkable changes, by which it is con- 

 taminated and rendered unfit for another inspiration, 

 unless it is renewed. f 



* The antiquity of the notion that air is the pabulum vita, is seen in the 

 book de Flatibus, usually ascribed to Hippocrates. The author regards the 

 aliment as three-fold victuals, drink, and air : but the latter he calls vital, 

 because we cannot dispense with a perpetual supply of it without danger to life. 



j- Consult Harvey's Dispute, upon the necessary renovation of the aerial succus 

 nlwilis, with the celebrated Astronomical Professor, J. Greaves, in the latter's 

 Description of the Pyramids hi Egypt, p. 101 sq. Lond. 1646. 8vo. Also the 

 immortal and popular Edm. Halley's Discourse concerning the means of fur- 

 nishing air at the Bottom of the Sea in any ordinary Depths. Phil. Trans.. 

 vol. xxix. No. 349. p. 492 sq. 



