on muscular Motion . 
2 » 
luntary muscles are each acted upon by different substances, 
which appear to be their peculiar stimuli ; and these stimuli 
co-operate with the sensorial influence in producing their con- 
tractions : for example, the bile appears to be the appropriate 
stimulus of the muscular fibres of the alimentary canal below 
the stomach, because the absence of it renders those passages 
torpid. The digested aliment, or perhaps the gastric juice in a 
certain state, excites the stomach. The blood stimulates the 
heart, light the iris of the eye, and mechanical pressure seems 
to excite the muscles of the oesophagus. The last cause may 
perhaps be illustrated by the instances of compression upon 
the voluntary muscles, when partially contracted, of which 
there are many familiar examples. Probably the muscles of 
the ossicula auditus are awakened by the tremors of sound ; 
and this may be the occasion of the peculiar arrangement ob- 
servable in the chorda tympani, which serves those muscles. 
These extraneous stimuli seem only to act in conjunction 
with the sensorial power, derived by those muscles from the 
gangliated nerves, because the passions of the mind alter the 
muscular actions of the heart, the alimentary canal, the respi- 
ratory muscles, and the iris ; so that probably the respective 
stimuli already enumerated, only act subserviently, by awak- 
ening the attention of the sensorial power, ( if that expression 
may be allowed,) and thereby calling forth the nervous in- 
fluence, which, from the peculiar organization of the great 
chain of sympathetic nerves, is effected without consciousness : 
for, when the attention of the mind, or the more interesting 
passions prevail, all the involuntary muscles act irregularly, 
and unsteadily, or wholly cease. The movements of the iris of 
the common parrot is a striking example of the mixed influence. 
The muscles of the lower tribes of animals, which are often 
