262 OBSERVATIONS 
done, that both rows of intercoital mufcles were in action du= 
ring infpiration. 
Arrer proving, that both rows of intercoftal mufcles con- 
fpire in elevating the ribs, I ufed to point out the fallacy of the 
demonttrations, by which BAYLE, HAMBERGERUS, and others, 
have pretended to prove, that the internal intercoftal mufcles 
deprefs the ribs. The machine they defcribe as reprefenting 
the ribs, vertebre and fternum, refembles very exaGtly two 
wooden rulers A, B, kept 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, patling obliquely from the one ruler to the 
other, and decuffating each other. See T. 2. fig. 9. 
Let C, one of the pieces of brafs, reprefent the vertebrz, 
and the other piece D the fternum. 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. 
Tuen, let C, reprefenting the immoveable vertebrz, be held 
faft, and let EF be pulled or thortened, 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 mutt 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. 
Bur 
