334 ESSAYS anp OBSERVATIONS 
feeling in animals merely by ating on the 
nerves to which it is applied (c) (d), it 
follows, that the nerves are the inftru- 
ments of fenfation, or, at leaft, neceflary 
to it. Nor is it fufficient to deftroy this 
conclufion, that there have been inftances 
of animals endowed with feeling whofe 
brains were fo greatly difeafed, as to feem 
incapable of performing their functions. 
It is far from being fafe to build theories 
in phyfic upon a few monftruous appear- _ 
ances in nature. 
(g) Ir appears from N° 4. and 5. com- 
pared with N°3. 6.8. ro. and 11. thatde- 
collation and the deftruction of the fpinal 
marrow does not weaken or deftroy the 
heart’s motion in frogs, near fo foon as 
opium injected into their ftomach and 
guts, or applied to the mufcles and bow- 
els of the lower belly and thorax, 
(4) AtTHo’ a folution of opium ap- 
plied to the opened thorax and abdomen 
of a frog, after decollation and the de- 
ftruction of its {fpinal marrow, foon wea- 
kens or deftroys the motion of the heart; 
yet it does not produce thefe effects fo — 
{peedily 
