OF RESPIRATION. 03 



thorax when cutting away a cancer, lie saw the heart through 

 the pericardium.* 



(B) Although each lower rib must execute a greater extent of 

 motion from being longer than the one above, yet the first is 

 asserted by M. Majendie to be absolutely more moveable than 

 the second, the second than the third, &c. : and this because the 

 first has but one articular surface, is articulated with but one 

 vertebra, and possesses neither internal nor costo-transverse liga- 

 ment, and has the posterior ligament horizontal, and because 

 slight shades of difference exist in the disposition of the liga- 

 ments of the six other ribs.f 



(C) To Dr. Carson we are indebted for the best account of 

 the mechanical part of respiration. 



The substance of the lungs is highly elastic, and constantly 

 kept in a foi ced state of distention after birth by the pressure of 

 the atmosphere. This is evident, as upon puncturing the walls 

 of the thorax, the lungs instantly collapse, — a circumstance 

 arising from the atmospheric pressure on the one hand becoming 

 counterbalanced on the other, so that their elasticity, expe- 

 riencing no opposition, becomes effective. During inspiration, 

 the intercostal muscles raise and draw out the ribs, and the dia- 

 phragm descends : the enlargement of the thoracic cavity is . 

 instantly followed of necessity by the greater distention of the 

 substance of the lungs from the diminished resistance to the 

 atmosphere gravitating in the bronchiae. The diaphragm and 

 intercostal muscles ceasing to act, the substance of the lungs 

 exerts its elasticity Avith effect, recovers its former dimensions, 

 and drives out the additional volume of air just admitted, anil 

 the passive diaphragm and intercostal muscles follow the shrink- 

 ing substance of the lungs, offering, from their relaxation, no 

 resistance to the atmosphere pressing on the surface of the chest 

 and abdomen. Thus expiration is produced. The muscular 



* Journal de Mddecinc. 1818. 



t Precis Ettmentabf. Tome ii. p. 270. 



