262 OBSERVATIONS 



done, that both rows of interco.tal mufcles were in action du- 

 ring infpiration. 



After proving, that both rows of intercoftal mufcles con- 

 fpire in elevating the ribs, I ufed to point out the fallacy of the 

 demonftrations, by which Bayle, Hambergerus, and others, 

 have pretended to nrove, that the internal intercoftal mufcles 

 deprefs the ribs. The machine they defcribe as reprefenting 

 the ribs, vertebrae and flernum, refembles very exactly two 

 wooden rulers A, B, kent parallel by two pieces of brafs, C 

 and D, fuch as are ufed for drawing parallel lines ; and the 

 two layers of the intercoftal mufcles are reprefented by the 

 threads EF and HG, palling obliquely from the one ruler to the 

 other, and d< j cuffating each other. See T. 2. fig. 9. 



Let C, one of the pieces of brafs, reprefent the vertebrae, 

 and the other piece D the flernum. Let A reprefent the upper- 

 moft rib on the right fide of the body, and B the fecond rib. 

 Let EF reprefent the external, and HG the internal intercoftal 

 mufcle. 



Then, let C, reprefenting the immoveable vertebrae, be held 

 faft, and let EF be pulled or ihortened, they tell us, that the 

 fecond rib B muft be more affected than the firft, becaufe the 

 lower end of the mufcle being at a greater diftance from C than 

 the upper end of it, the mufcle will act upon the fecond rib 

 with a longer lever, and therefore that the external intercoftal 

 mufcles muft elevate the rib. 



But when the internal intercoftal mufcles, reprefented by 

 HG, are fhortened, they obferve, that matters will be reverfed ; 

 and as their origin in the firft rib is farther from the vertebra, 

 or centre of motion, than their infertion in the fecond rib, 

 that having a longer lever, they muft ferve to pull the firft rib 

 down. 



Accordingly, the rulers, on pulling alternately the threads 

 EF and HG, will be moved alternately upwards and downwards. 



But 



