RESPIRATION. 203 



" It is a considerable muscle, and, in the words of Haller, next 

 in importance to the heart. Its utility in the mechanical part of 

 respiration was long since shown, by the excellent experiments 

 of Galen * upon living animals, to depend chiefly on the phrenic 

 nerve.u 



" Its antagonists are the abdominal muscles, especially the two 

 sets of oblique and the transverse. 



" The thorax, thus constituted, is, after birth, dilated by in- 

 spiration, and subsequently reduced to a smaller capacity by ex- 

 piration. 



" During the former act, the thorax is enlarged laterally and in- 

 feriorly, so that the bodies of the six ribs mentioned above are 

 elevated and their inferior margin drawn somewhat outwards ; the 

 arch of the diaphragm is at the same time rather depressed and 

 flattened." 



The lungs in ordinary respiration do not descend lower than 

 the sixth rib, and the lateral portions of the diaphragm, ascend- 

 ing into the empty space, lie in contact on both sides with the 

 lower ribs, each covered by its costal pleura. 



Dr. Carson gives the following account of the mechanical part 

 of respiration. 



The substance of the lungs is highly elastic, and constantly 

 kept in a forced state of distension after birth by the pressure of 

 the atmosphere. 31 This is evident also from the lungs collapsing 

 upon our puncturing the walls of the thorax, a circumstance 

 arising from the atmospheric pressure on the one hand becoming 

 counterbalanced on the other, so that their elasticity, experiencing 

 no opposition, becomes effective.? During inspiration, the inter- 

 costal muscles raise and draw out the ribs, and the diaphragm 

 descends : the enlargement of the thoracic cavity is instantly fol- 

 lowed of necessity by the greater distension of the substance of 

 the lungs from the diminished resistance to the atmosphere gra- 



1 " De Anatomicis Administrationibus, 1. viii. cap. 8. The whole book is very 

 rich in experiments on respiration." 



u " Ephr. Kriiger, De nervo phrenico. Lips. 1759. ; reprinted in Sandifort's 

 Thesaurus, torn. iii. 



Walter, Tab. nervor. thorac. el abdominis, tab. i. fig. 1. n. 1." 



x See Haller, El. Phys. lib. viii. s. iv. pp. 259. 275. 



y Dr. Carson found the elasticity of the lungs of calves, sheep, and large dogs 

 balanced by a column of water of from a foot to a foot and a half in height, and 

 of rabbits and cats by a column of from six to ten inches. Phil. Trans. 1820. 

 Part 1. 



