IRRITABILITY. 
must soon cease in consequence, from want of 
a supply of blood. 
_ These facts prove that arterial blood is the 
‘necessary stimulus of the left side of the heart, 
‘its irritability being low ; but that venous blood 
isa sufficient stimulus of the right, from its 
higher irritability : the phenomena plainly flow 
‘from the law, that the quantity of respiration 
and the degree of irritability observe an in- 
verse ratio to each other, and from the facts on 
which that law is founded. In this double 
‘sense, besides that of distinct cavities, the 
“Mammalia have, therefore, two hearts; and as 
‘the highly aerated blood of the left is the pecu- 
liar property of birds and the mammalia, so 
the highly irritable fibre of the right may be 
ae to that of the heart of reptiles and 
the fishes. 
Except for the objection to new terms, the 
left side of the heart might be termed arterio- 
contractile, and the right veno-contractile ; the 
first being stimulated by arterial, the second 
by venous blood. 
It is quite obvious that the heart will bear a 
‘suspended respiration better, the more nearly 
‘its irritability approaches to that which may be 
‘designated veno-contractile. The power of 
bearing a suspended respiration thus becomes 
a@ measure of the irritability. It is expressed, 
numerically indeed, by the length of time 
during which the animal can support a sus- 
pended respiration ; a conclusion of the highest 
degree of importance in the present inquiry. 
' Birds die almost instantly on being sub- 
‘merged in water ; the mammalia survive about 
three minutes, the reptiles and the batrachia a 
“much greater length of time. 
_ The unborn fetus, the young animal born 
‘with the foramen ovale open, the reptile, the 
‘ollusca, having all a state of the heart ap- 
‘proaching to the veno-contractile, bear a long- 
‘continued suspension of the respiration, com- 
‘pared with the mature animal of the higher 
classes. 
_ But the most remarkable fact deducible from 
this reasoning is the following: if such a case 
isted as that of the left side of the heart 
being nearly or absolutely veno-contractile, such 
‘an animal would bear the indefinite suspension 
of respiration ; such an animal would not drown 
though immersed in water. Now there is pre- 
‘cisely such a case. It is that of the hyberna- 
‘ting animal. It may be shown that in the 
state of perfect hybernation the respiration 
is nearly suspended ; the blood must, there- 
fore, be venous. See Hysernation. Yet the 
heart continues to contract, although with a 
‘reptile slowness. The left ventricle is, there- 
fore, veno-contractile, and in this sense, in fact, 
‘sub-reptile. The case forms a solitary excep- 
‘tion to the law pointed out by Harvey, that the 
left ventricle ceases to contract sooner than the 
‘right. If in the hybernating animal the left 
_Ventricle does cease to beat sooner than the 
right, it is only in so slight a degree as to be 
referred to the greater thickness of its parietes, 
_and the slight degree in which respiration still 
remains. It is obvious that the foregoing state- 
ment must be taken with its due limitations. 
35 
Venous blood is unfit for the other animal pur- 
poses, even though it should stimulate the 
heart to contraction. 
Another mode of determining the degree of 
irritability, is the application of stimuli, as 
galvanism. A muscular fibre endued with 
high irritability, as that of the frog, and the 
galvanic agency are mutually tests of each 
other.* 
A third criterion and measure of the irrita- 
bility is afforded by the influence of water at 
temperatures more or less elevated, in in- 
ducing permanent contraction of the muscular 
fibre. 
There are two other properties of animals 
which depend upon the varied forms of the 
inverse ratio which exists between the respira- 
tion and the irritability. The first is activity, 
the second, tenacity of life. 
The activity, which, I believe, M. Cuvier 
has confounded with the irritability, is generally 
directly proportionate to the respiration, and 
intimately depends upon the condition of the 
nervous system resulting from the impression 
of a highly arterial blood upon its masses, and 
not upon the degree of irritability of the muscu- 
lar fibre. It is the pure effect of high stimulus. 
To show that M. Cuvier has blended the 
idea of the irritability of the muscular fibre 
with that of the activity of the animal, it is 
only necessary to recur to the passages already 
quoted from that author, and to adduce the 
observations with which they are connected. 
“ On vient de voir a quel point les animaux 
vertébrés se ressemblent entre eux; ils offrent 
cependant quatre grandes subdivisions ou 
classes, caractérisées par l’espece ou la force 
de leurs mouvements, qui dépendent elles- 
mémes de la quantité de leur respiration, at- 
tendu que c’est de la respiration que les fibres 
musculaires tirent l’énergie de leur irritabilite.”’+ 
“ Comme c’est la respiration qui donne au 
sang sa chaleur, et a la fibre la susceptibilité 
pour l’irritation nerveuse, les reptiles ont le 
sang froid, et les forces musculaires moindres 
en totalité que les quadrupédes, et a plus forte 
raison que les oiseaux; aussi n’exercent-ils 
guére que les mouvements du ramper et du 
nager ; et, quoique plusieurs sautent et courent 
fort vite en certains moments, leurs habitudes 
sont généralement paresseuses, leur digestion 
excessivement lente, leurs sensations obtuses, 
et dans les pays froids ou tempérés, ils passent 
presque tous l’hiver en léthargie.” { 
It is extraordinary that M. Cuvier should 
have associated the elevated temperature of the 
blood with a high irritability of the muscular 
fibre, when they are uniformly separated in 
nature, and are, indeed, absolutely incompa- 
tible in themselves. The muscular fibre of the 
frog is so irritable, that it would instantly pass 
into a state of rigid contraction, if bathed with 
a fluid of the temperature of the blood of birds.§ 
* Bostock on Galvanism, pp. 4, 14. 
+ Le Régne Animal, tome i, pp. 56, 57, 2de 
edit. 
t Ibid. tome ii. pp. 1, 2. 2de edit. 
§ See an Essay on the Circulation, chap. vii. 
pp. 180, 181. 
D2 
