WHICH SUBSISTS BETWEEN THE RESPIRATION AND IRRITABILITY. 323 
The egg, the foetus, the tadpole, the larva, &c. are respectively animals of 
lower respiration, and of higher irritability, than the same animals in their 
mature and perfect 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 physiological con¬ 
dition are invariably from higher to lower. 
These views are further illustrated by a reference to the quantity of stimu¬ 
lus and the degree of irritability of each of the parts and organs of the animal 
system. But it is to the quantity of respiration, and the degree of irritability 
of the heart, that our attention is to be principally directed at this time. 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 stimu¬ 
lus, as it is of temperature and nutriment, to the various parts of the system. 
As oxygen is the principal stimulus, the heart is the principal organ of irrita¬ 
bility, in all the vertebrated animals ; if the contact of oxygen be interrupted, 
all perish in a greater or less period of time. 
The extraordinary differences which exist in animals which occupy different 
stations in the zoological scale, have long excited the attention 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 remark applies to that singular state of existence and of the functions 
of life, designated hybernation. But it appears to me that a sufficiently com¬ 
prehensive view has not been taken of the subject, and that many facts, with 
their multitudinous relations, still require to be determined. 
I. Of the Pneumcitometer. 
The principal of these facts is that of the quantity of respiration. This is 
greater in proportion as the animal occupies a higher station in the zoological 
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 
