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 hatched without feathers, and of certain animals which are bons blind; and in hybemation it is almost extinct.
To ascertain the quantity of respiration in any given animal, with extreme minuteness, was a task of great difficulty. It was still
more difficult to determine this problem, so as to represent the quantities of respiration in the different kinds, ages, and states of animals, in an accurate series of numbers. The changes induced in a given volume of air made the subject of experiment, by changes in the tem perature and pressure of the atmosphere, and by variations in the height of the fluid of a pneumatic trough, which it is so difficult to appreciate minutely ; the similar changes in duced by the humidity of expired air, and by the heat of the animal itself, were so many and complicated, that it appeared almost im possible to arrive at a precise result. These difficulties, in fine, were such as to lead one of the first chemists of the present day to give up some similar inquiries in despair.
Fortunately I have been enabled to devise an apparatus which reduces this complex pro blem to the utmost degree of simplicity. I now beg the indulgence of the reader whilst I give a detailed description of its construction and mode of operation.
This apparatus, which I shall designate the pneumatometer, consists of a glass jar (fig. 1, a, b,) inverted in a mercurial trough (c , d,) so grooved and excavated, as accurately to receive the lower rim of the jar and the lowest part of the tube (e,f, g,) and also to admit of the ani mal which is made the subject of experiment, being withdrawn through the mercury. This jar communicates, by means of the bent tube (e, f, g, h,) with the gauge (i,j,) which is in serted into a larger tube (k, 1,) containing water. A free communication between the jar and the external air is effected and cut off, at any time, by introducing and withdrawing the little bent tube (m, a,) placing the finger upon the ex tremity (rn,) whilst the extremity (n) is passed through the mercury.
If the jar be of the capacity of one hun dred cubic inches, the gauge is to contain ten, and to be graduated into cubic inches and tenths of a cubic inch ; so that each smallest division shall be the thousandth part of the whole contents of the jar.