PHYSIOLOGY. 
338 
medulla of the brain and nerves, but alfo at the fame time 
that living principle, or Jpirit of animation, which refides 
throughout the body, and which we perceive only through 
its efre< 5 fs. An idea is defined to be a motion of the 
fibres of fome immediate organ of fenfe ; and hence is 
frequently termed alfo a fenfualmotion. Perception com¬ 
prehends both the fibrous motion or idea, and the atten¬ 
tion to it. When the pain or pleafure arifing from this 
motion and this attention produces other fibrous motion, 
it is termed fenjation; thus limiting this term to an 
active fenfe. Ideas not immediately excited by external 
obje&s, but which recur without them, are termed either, 
i. Ideas of recolledion, as when we will to repeat the 
alphabet backwards; or 2. Ideas of J’uggeJlion, as when 
we repeat it forwards, A fuggefting B, B fuggefting C, 
See. from habit. 
After mentioning a number of experiments to prove 
the fibrous motions of the organs of fenfe, Dr. Darwin 
proceeds to lay down the following laws of animal caufa- 
tion. 1. The fibres, which conftitute the mufcles and 
organs of fenfe, poffefs a power of contraction. The cir- 
cutnftances attending the exertion of this power of con- 
tra&ion constitute the laws of animal motion, as the cir- 
cumftances attending the exertion of the power of at¬ 
traction conftitute the laws of inanimate matter. 2. The 
Jpirit of animation is the immediate caufe of the contrac¬ 
tion of animal fibres. It refides in the brain and nerves, 
and is liable to-general or partial diminution or accumu¬ 
lation. 3. The ftimulus of bodies external to the moving 
organ is the remote caufe of the original contractions of 
animal fibres. 4.. A certain quantity of ftimulus pro¬ 
duces irritation, which is an exertion of the fpirit of ani¬ 
mation exciting the fibres to contraction. 5. A certain 
quantity of contraction of animal fibres, if it be perceived 
at all, produces pleafure; a greater or lefs quantity of con¬ 
traction, if it be perceived at all, produces pain. Thefe 
conftitute fenjation. 6. A certain quantity of fenfation 
produces ilefire or averfon. Thefe conftitute volition. 7. 
All animal motions which have occurred at the fame time 
or in immediate fucceflion, become fo connected, that, 
when one of them is reproduced, others have a tendency 
to accompany or fucceed it. When fibrous contractions 
fucceed or accompany other fibrous contractions, the con¬ 
nection is termed ajj'ociation; when fibrous contractions 
fucceed fenforial motions, the connection i? termed cau- 
Jation ; when fibrous and fenforial motions reciprocally in¬ 
troduce each other, it is termed catenation of animal motions. 
All thefe connections are faid to be produced by habit; that 
is, by frequent repetition. Thefe law£ of animal caufa- 
tion are,'according to our author, evinced by numerous 
fafits, which occur in our daily exertions, and are em¬ 
ployed by him to explain the difeafes and decay of the 
animal fyftem. 
The four fenforial powers, upon which all the aCtions 
or motions depend, are thus characterized: Irritation is 
an exertion or change of fome extreme part of the fenfo- 
rium, refuting in the mufcles or organs of fenfe, in con¬ 
fequence of theappulfes of external bodies. Senfation is 
an exertion or change of the central parts of the fenfo- 
rium, or the whole of it, beginning in fome of thofe ex¬ 
treme parts of it which refide in the mufcles or organs of 
fenfe. Volition is an exertion or change of the central 
parts of the fenforium, or of the whole of it, terminating 
in fome of thofe extreme parts of it which refide in the 
mufcles or organs of fenfe. Ajj'ociation is an exertion or 
change of fome extreme part of the fenforium, refiding in 
the mufcles or organs of fenfe, in confequence of fome an¬ 
tecedent or attendant fibrous contractions. 
To thefe four faculties correfpond fo many clafies of 
fibrous contractions, named irritative, JenJitive, voluntary, 
■and ajfociate. But all mufcular motions, and all ideas, 
are originally irritative, and become caufable by fenfation 
and volition from habit, i.e. becaufe pleafure or pain, or 
defire or averfion, have accompanied them ; thole ideas 
or mufcular motions which have been frequently excited 
1 
together, ever afterwards have a tendency to accompany 
each other. 
Of thefe motions, the ajfociate feem moft to have ex¬ 
cited Dr. Darwin’s attention. He divides them into 
three kinds: irritative ajfociations, as when any part of the 
extraCied heart of a frog is irritated by puncture, the 
whole heart contracts regularly •, fenf live ajfociations, or 
the trains or tribes of motions eftablilhed by pain or plea¬ 
fure ; and voluntary ajfociations, or thofe produced by vo¬ 
lition. The activity of this power of volition is fuppofed 
to form the great difference between man and the brute 
creation; the means of producing pleafure and avoid¬ 
ing pain given to man by this power being denied to 
brutes. 
Correfponding to thefe four clafies of motions, there 
are four clafies of ideas: irritative, preceded by irrita¬ 
tion ; JenJitive, preceded by the fenfation of pleafure 
or pain; voluntary, preceded by voluntary exertion; 
and aJJ'ociute, preceded by other ideas or mufcular mo¬ 
tions. 
It has been obferved in Hudibras, that 
----A rhetorician’s rules 
Serve nothing but to name his tools. 
So we find that a confiderable part of Darwin’s work is 
taken up in eftablilhing the new meaning which he 
attaches to terms well underftood and long adopted. 
The hypothelis of Cuvier (as it feems to us a very 
ridiculous one) is, that fenfation is produced by a change 
in the fubftance of the brain and nerves. M. Cuvier 
illuftrates this doftrine in the following manner. 
The nervous fyftem is fufceptible of two kinds of 
aftion; one which is confined to our fenlitive faculty, and 
another which affeCts our vital and vegetative functions 
only. External fenfations are produced by the impref- 
fions of external bodies on ourfenfes; internal fenfations, 
by changes which take place in the ftate of the internal 
parts of the body to which the nerves are diftributed; and 
fpontaneous fenfations are caufed by a change in the 
nerves, or in the brain itfelf, without any external ex¬ 
citement. Thefe circumftances, added to the phenomena 
arifing from the cutting or tying of nerves, fliow, that 
fenfation does not refide in the external organs, but 
nearly in the centre of the nervous fyftem; and that the 
external organs ferve only to receive the aflion of the 
external bodies, and to convey it to the nerves, by which 
it is propagated to a greater diftance. They alfo demon- 
ftrate, that this propagation is not produced by any 
matter or concuftion, but by a change in the ftate of the 
nervous fubftance. This change may arife from internal 
caufes, or it may be produced by external caufes different 
from thofe which ufually occafion it. The nerves are 
not merely pafiive agents, nor the conductors or refervoirs 
of any particular matter; but it appears, that the fub¬ 
ftance which produces fenfation is liable to be confumed, 
or to lofe its activity by exertion. 
There are phenomena which fliow that the general 
fufeeptibility of the nerves for receiving fenfations may 
vary in confequence of caufes external to the nerves 
themfelves, and which can operate only by altering 
their fubftance. Certain medicines weaken or revive 
that fufeeptibility ; inflammation frequently increafes it 
to an exceflive degree. Does this take place in confe¬ 
quence of an increafed fecretion of the nervous matter? 
The moft remarkable change that occurs in the fulcepti- 
biliiy of nerves, is fleep. It is not unnatural to fuppofe 
that this change may be occafioned by the temporary 
lofs of the fubltance which is eflentially fenfitive. But 
how does it happen that fleep depends, in a certain de¬ 
gree, on the will? Why do we awake fuddenly, or 
from caufes which do not appear calculated to reftore 
that fubftance? Why does cold produce fleep? From 
thefe obfervations, may it not rather be fuppofed that 
this ftate is the eftefil of a change in the chemical nature 
of the nervous fubftance ? 
But, 
