IRRITABILITY. 31 
farum intestina sunt, que et evulsa pergunt se 
contrahere et frigida demum; etiam his tena- 
cius cor, si omnia conputaveris, in pullo etiam 
evidentissime et in frigidis animalibus.” * 
The observations of Nysten are more exten- 
sive, and his inferences were deduced from expe- 
riments made upon the human subject imme- 
diately after decapitation. They areas follow: 
“1. La contractilité du ventricule aortique 
était éteinte 49 minutes apres la mort; 
“2. L’aorte n’a offert aucun mouvement de 
contraction ; 
_ 3. Cinquante-six mimutes apres la mort, 
la contractilité de l’estomac, des intestins et de 
ta vessie était éteinte ; mais ces organes n’ont 
u étre soumis assez promptement au galvan- 
isme pour connaitre la durée rélative de leur 
force contractile ; 
“ 4. Le ventricule pulmonaire perdit sa con- 
tactilité une heure 58 minutes apres la mort ; 
** 5. Deux heures 2 minutes apres la mort, 
le diaphragme ne se contractait plus; les mus- 
cles de l'appareil locomoteur perdirent succes- 
_sivement leur contractilité & mesure que le 
contact de l’air agissait sur eux ; mais ceux qui 
ne furent exposés a l’air que tard, par exemple 
au bout d’environ 4 heures, ne cessérent de se 
_ mouvoir que 4 heures 15 minutes aprés la mort ; 
6. Les oreilletes du ceeur, qui étaient 
_ exposées a l’air depuis le commencement de 
lexpérience, ne cessérent de se contracter que 
_ 4heures 40 minutes apres la mort.” + 
But if there be a difference in the irritability 
_ of different organs in the same animal, there is 
a still greater difference in the different animals 
_ themselves of the zoological scale. It may be 
_ Stated in general terms, that the degree of the 
_ irritability in the different parts of the animal 
_ series, as tested by galvanism, is inversely as the 
_ quantity of the respiration; so that in the 
reptile tribes, in which the respiration is exceed- 
_ ingly low, the irritability of the muscular fibre 
is such as to afford a delicate test of galvanism ; 
and in birds, in which the respiration is at its 
maximum, the irritability exists in its lowest 
degree. 
This important subject deserves the fullest 
development. We shall here, therefore, insert 
some observations which were read to the 
_ Royal Society, and published in the Philoso- 
phical Transactions in 1832. 
The due actions of life, in any part of the 
zoological series, appear to depend upon the 
_ due ratio between the quantity of atmospheric 
_ change induced by the respiration, and the 
_ degree of irritability of the heart: if either be 
unduly augmented, a destructive state of the 
_ functions is induced ; if either be unduly di- 
_Minished, the vital functions languish and 
_ eventually cease. If the bird possessed the 
_ degree of irritability of the reptile tribes, or 
_ the latter the quantity of respiration of the 
former, the animal frame would soon wear out. 
If, on the contrary, the bird were reduced to 
the quantity of respiration appropriate to the 
ie i — See * 
* Haller, Prime Linez, 1767, p. 207. 
+ Recherches de Physiologic, {gn, p- 312, 
reptile, or the latter to the degree of irritability 
which obtains in the former, the functions of 
life would speedily become extinct. Various 
deviations from the usual proportion between 
the respiration and the irritability, however, 
occur, but there is an immediate tendency to 
restore that proportion ; increased stimulus ex- 
hausts or lowers the degree of irritability, 
whilst diminished stimulus allows of its aug- 
mentation. The alternations between activity 
and sleep afford illustrations of these facts. 
Changes in anatomical form in the animal 
kingdom present other illustrations of the law 
of the inverse proportion of the respiration 
and irritability. The egg, the foetus, the tad- 
pole, the larva, &c. are respectively animals of 
lower respiration, and of higher irritability, 
than the same animals in their mature and per- 
fect state. Changes in physiological condition 
also illustrate the same law. The conditions 
of lethargy, and of torpor, present examples 
of lower respiration, and of higher irritability, 
than the state of activity. 
It may be remarked that whilst changes in 
anatomical form are always from lower to higher 
conditions of existence, changes in the phy- 
siological condition are invariably from higher 
to lower. 
These views are further illustrated by a re- 
ference to the quantity of stimulus and the 
degree of irritability of each of the parts and 
organs of the animal system. The oxygen of 
the atmospheric air is the more immediate and 
essential stimulus of this organ. Taken up in 
respiration, it is brought into contact with the 
heart, by means of the blood, which may be 
considered as the carrier of this stimulus, as 
it is of temperature and nutriment, to the 
various parts of the system. As oxygen is 
the principal stimulus, the heart is the prin- 
cipal organ of irritability, in all the verte- 
brated animals; if the contact of oxygen be 
interrupted, all perish in a greater or less pe- 
riod of time. 
The extraordinary differences which exist in 
animals which occupy different stations in the 
zoological scale, have long excited the atten- 
tion of naturalists. Nor have the differences 
which obtain in the various ages and states of 
its existence, in the same animal, escaped the 
attention of the physiologist. A similar re- 
mark applies to that singular state of existence 
and of the functions of life, designated hyber- 
nation. But it appears to me that a sufficiently 
comprehensive view has not been taken of the 
subject, and that many facts, with their mul- 
titudinous relations, still require to be deter- 
mined. 
I. Of the pneumatometer.— The principal 
of these facts is that of the quantity of respi- 
ration. ‘This is greater in proportion as the 
animal occupies a higher station in the zoolo- 
gical scale, being, among the vertebrated 
animals, greatest of all in birds, and lowest 
in fishes ; the mammalia, the reptiles, and the 
amphibia occupy intermediate stations. The 
quantity of respiration is also remarkably low 
in the very young of certain birds which are 
