196 



CHAP. XII. 



RESPIRATION, AND ITS PRINCIPAL USE. 



f< THE lungs a , closely connected with the heart both by prox- 

 imity and by relation of function, are two viscera," of the shape 

 and size of the thorax, which they fill ; with a very large base, like 

 a horse's hoof; the right divided into three lobes, the left into two ; 

 " so light as to swim in water, and composed of a spongy, but 

 pretty tenacious and elastic b , parenchyma." 6 



Like other cavities, the chest is lined by a serous membrane, 

 by which is meant a close sac, translucent d during life and health, 

 and coherent by cellular membrane externally with every thing in 

 contact, with parietes and viscera; internally smooth, unattached, 

 in contact only with the opposite portions of itself, and moist- 

 ened by serum. 6 A serous membrane thus affords an external coat 

 to viscera, insulates them, and facilitates their movements. 



Each lung has a serous membrane, called Pleura, so that the 

 pleurae are two closed sacs, one of which lies over each lung, 



a " Soemmerring and Reisseisen, uber die Stru.ctur, die Verrichtung und den 

 Gebrauch der Lungen. Zwey Preischriften. Berlin, 1808. 8vo." 



b " J. Carson, On the Elasticity of the Lungs, in the Phil. Trans. 1820. p. 29. 



Consult also, Const. Ern. de Welzien, De Pulmonum avtenergia, $fc. Dorpat, 

 1819. 8vo." 



c " Respecting all the organs concerned in respiration, consult Corn. J. Van 

 Den Bosch, Anatomia Systematic Resptrationi inservientis Pathologica. Harlem, 

 1801. 4to. pp. 1 44." 



d M.Richerand tells us, that, on removing a portion of the thorax when cutting 

 away a cancer, he saw the heart through the pericardium. Journal de Medecine t 

 1818. 



e Dr. Marshal, from many experiments, believed that this is not the case, but 

 that, whenever fluid is discovered, we must regard it as the effect of either disease 

 or the struggle of dying. His experiments were made on the ventricles of the 

 brain, the theca vertebralis, the pleura, and the pericardium {The Morbid Ana- 

 tomy of the Brain in Mania, Hydrophobia, fyc.} ; yet, when Dr. Magendie ha& 

 opened the membranes of the brain or spinal marrow, I have myself seen a 

 colourless clear fluid instantly escape. 



