SECT. VI. PHYSIOLOGY. 39 
the ovum to the last expiration, incessant decom- 
position is going forward in the human body. 
The vital force is the cause of decomposition in 
the animal economy ; vitality must, therefore, on 
this principle, be a compound substance like 
electricity. 
Lxxiu. It was reasoning upon this principle, 
that induced Dr. Sillar and myself to put pieces 
of meat into a wound made in a dog’s thigh, and 
to try if the vital force would decompose them. 
The success of the experiments is a strong pre- 
sumption in favour of the reasoning which 
pre-supposed the results which were afterwards 
obtained on trial, and which will be related in 
the section on digestion. 
Lxxiv. Muscular contraction not being the 
mere absence of expansion, is also another proof 
that there must be a negative contractile, as well 
as a positive expansive vital force. 
Lxxv. A theory of irritability, formed on the 
compound nature of vitality, would refer expan- 
sion of the whole fibrous structure of the body to 
the positive vital force, and contraction of the 
same to,the negative vital force which is commu- 
nicated to the voluntary muscles by the mind, 
and to the vascular system and intestinal canal, by 
their contents. If this theory do not afford a clue 
to voluntary contraction of the muscles, I. know 
not what will: it has at least the merit of several 
