Different Animals, &c. 519 
to believe are constantly and invariably distri- 
buted to every muscular fibre. —It has been 
urged, as an argument in favour of a vis insita, 
that muscles, or muscular fibres, will contract for 
a considerable time, after their separation from 
the body, on the application of a stimulus.* But 
this fact may be perfectly explained by the ad- 
mission of a nervous power only; for why can- 
not the nerves, which evidently possess an energy 
independently of the brain, retain this energy 
after the removal of muscles from the body, as 
long as the muscular fibres can retain their sup- 
posed vis insita? 
From what has been stated in the preceding 
part of this paper, the following conclusions 
amongst others may, I conceive, be very fairly 
drawn. 
1. That every perfect animal, from man_to 
the polypus, possesses the powers of sensation 
and voluntary motion, 
2. That infants, though born destitute of 
brain, or even of brain and spinal marrow, pos- 
sess these two important faculties. 
g. That the fetus in utero is neither destitute 
of sensation, nor voluntary motion, 
* In warm-blooded animals this takes place for an 
hour or more; in cold-blooded ones more than a day 
pfterwards, 
