Bauhaus-Universität Weimar

OTHER INVOLUNTARY MOTIONS. 17 
a paflage ; or becaufe it was thought that the fpirits could only be 
difcharged by drops, and not in an equable dream, from the ex¬ 
tremities of fuch fubtile tubes as the nerves were conceived to be *. 
Various other hypotheles were framed to explain the alternate 
motion of the heart ; a problem not lefs difficult than curious ! 
Thefe I ffiall pafs over in ffience, leaving them to fall by their own 
infufficiency, or the arguments of others ; and content myfelf with 
mentioning the defeds of fome of the later fyflems, which, from 
their plaufibility, or the high character of their authors, are in- 
titled to the greateft regard. 
The theory of the motion of the heart which has of late years 
met with the moil favourable reception, is that of the celebrated 
Boerhaave, who deduces the alternate fyflole and diciflole of that 
mufcle chiefly from the peculiar circumftances of the cardiac 
nerves ; for as the greatefl: part of thofe nerves paflTes between the 
auricles and large arteries of the heart, he concluded that they 
muft be comprefled at the end of every fyßole, when thefe cavities 
and veflels are greatly diflended with blood ; whence the motion 
of the fpirits being intercepted, the heart muft be rendered para¬ 
lytic ; but that whenever, upon the fubfequent contraction of the 
auricles and arteries, this compreffion ceafes, and the nerves tranf- 
mit their fluid as formerly, the heart muft contraCt anew f. 
This hypochefis, however ingenious, will appear infufficient, if 
we attend to the following confiderations. 
i. All the cardiac nerves do not pafs between the auricles and 
arteries. Not to mention many fmaller ones, there are two confider- 
able branches diftributed to the mufcular fubflance of the heart, 
which neither pafs between the two auricles nor the two arteries ; 
. and therefore cannot be liable to any alternate compreffion from 
them. 
C 2. I 
* Borell. de mot. animal. lib, u. cap. 6. prop, 79. 
t Inftitut. med. No, 409.
        

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