344 DR ALISON’S OBSERVATIONS ON THE PRINCIPLE OF VITAL AFFINITY. 
limit and define our notions of the powers exercised by the Nervous System, in 
producing the phenomena of the life of animals, maintaining on that subject the 
different parts of one general and fundamental proposition ; viz., that there is no 
good evidence, and that in the absence of such evidence it is unphilosophical to 
assume, that any changes in the nervous system are essentially concerned in pro- 
ducing any phenomena in the healthy state of the system, except those in which 
some mental act is necessarily involved ; but that all the powers which are exer- 
cised, in the natural and healthy state, by the nervous system, in a living body, 
are those by which it fulfils its destined office as the seat, and the instrument, of 
mental acts,—of Sensation, Thought, and Instinctive or Voluntary effort; and 
that the nature of these powers, and the uses or intention of the different parts, 
and of all the arrangements of the nervous system, if judged of simply in refer- 
ence to these, the specific objects of its creation, are tolerably well ascertained ; 
vindicating, at the same time, the doctrine of HALLER, in regard to the separate 
vital property of Irritability or Contractility in muscles, and its different modes 
of connection with the nervous system. 
I likewise stated, on a former occasion, to this Society the evidence of another 
fundamental principle in physiology—of the existence and the chief agencies of a 
power exercised by living bodies, and peculiar to their living state—which is 
capable of producing motion, or of influencing motion otherwise produced, but 
which acts in the way of Attraction and Repulsion; and is, therefore, quite dis- 
tinct from that living power of animal solids, acting in the way of contraction 
and impulse, which is well understood; and to which, since the time of HAaLuEr, 
the name of Irritability, or the more general term Contractility, has been applied. 
Although both these principles have been strongly contested, I have had the 
satisfaction of seeing them adopted, and their importance acknowledged, by most 
of those who have prosecuted the science of Physiology in this country of late 
years, with the greatest diligence and success. I have now laid before the So- 
ciety the general grounds of a third opinion, which I hold to be of equal rank in 
physiology ; viz., that there are laws, peculiar to living bodies, acting to a limited 
extent only, and already in a considerable degree ascertained, which alter and 
control the ordinary chemical Affinities of the matter composing those bodies, as 
distinctly as the laws of muscular contraction, or of vital attractions and repul- 
sions, modify the effects of the ordinary mechanical properties of matter within 
them. And if this doctrine shall, as I confidently expect, be equally admitted to 
be correct, then, although laying claim to no credit as a discoverer, I hope I may 
be allowed the satisfaction of reflecting, that I have contributed somewhat to- 
wards fixing the foundations of the noble science of Physiology ; and establish-— 
ing those principles in that science, to which continual reference must necessarily 
be made, in any speculations to which we can apply the epithet scientific, in re- 
gard either to the nature of diseases or the operation of remedies. 
