92 OF RESPIRATION. 



be remembered, will, in my opinion, explain tbc cele- 

 brated problem of Harvey* better f than most other 

 attempts of physiologists. (D)X 



NOTES. 



(A) A correct notion can scarcely be formal from this descrip- 

 tion. The pleura is two closed sacs, one of which lies over each 

 lung, one portion of the sac adhering closely to it, and one lying 

 over this again ; the internal surfaces of both portions are alwa) s 

 in contact, because, if the parietes of the thorax expand and 

 draw with them the external portion, the lung at the same time 

 expands with air and forces out the internal in the same degree. 

 It is commonly said that a portion of fluid (not vapour) exists 

 in serous membranes for the purpose of lubrication. The late 

 Dr. Marshal, however, proved 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 vertc oralis, the 

 pleura, and the pericardium. § 



These membranes during life and health are transparent. At 

 least M. Richerand tells us that on removing a portion of the 



• Wm. Harvey, De cirrnlat. tanguin. ad J. Riolan. p. 238. Gku-irov. 

 1751. 12mo. ; and especially his Exerc. de grner. Animaliutn. p. 263. Lond. 

 1651. 4to. 



t See TTicod. G. Aug. Roose, T'ber das ErUicken neugebohrner Kinder, in 

 his Physiologisch. 1'ittersuehnngen. Hrunsw. 1796. 8vo. J. D. Hcrholdt, 

 De rita imprimis foetus humani, ejusqite iiiurtc sub part u. Havn. 1802. 8vo. 



X Consult, for example, Petr. J. Daottstcnr, De llespiratione. Lugd. 1743. 

 4to. p. 54 »q. Rob. Wliytt, on the Vital and other involuntary motions of 

 animal,, p. 222. Edinb. 17.M. 8vo. 



i The ^forbid Anatomy nftlic Drain in Mania, Hydrophobia, tfc. 



