It is important that the animal should be left for a considerable time in the very situation in which it is to remain during the experiment, before that experiment is begun, and before the jar is placed over it. In this manner the effect of timidity or restlessness is allowed to subside, and prevented from mingling with that of the natural state of the respiration. A bit of cork must also be attached to the mercurial trough, so as to float upon the mercury at t, and pre vent the disturbing effect of the contact of this fluid with the animal.
It is also well, after having placed the jar in the groove of the mercurial trough, to pour a little water over the mercury exterior to the jar. The apparatus is thus rendered perfectly air-tight, which is not always effected by the mercury alone.
By means of this apparatus we readily and accurately determine the quantity of the re spiration of any given animal, in any given circumstances.
II. Qf the measure of the irritability.— The problem to be next determined is that of the degree of irritability of the muscular fibre, and especially of the heart. The question is beset with scarcely fewer or less difficulties than that of the quantity of respiration, whilst it involves far greater errors and more dis crepancy of opinion on the part of physio logists.
Even Baron Cuvier has fallen into these errors. It will be shortly demonstrated that the degree of irritability is, in every instance, inversely as the quantity of respiration. Yet M. envier, in a remarkable paragraph, states the very contrary, and even speaks of that which is the exhauster, as the repairer, of the irritability; whilst, on the other hand, he makes statements which appear to me at va riance with this very opinion. In the Ana tomic Compare (tome i. p. 49,) this cele brated writer observes, " Les experiences modernes ont montre qu'un des principaux usages de la respiration est de ranimer la force musculaire, en rendant it Ia fibre son irrita bilite epuisee." See also tome iv. p. 301. Similar observations are made in N. Cuvier's more recent work, the Rbgne Animal : " C'est de Ia respiration que les fibres musculaires tirent 1'Cnergie de leur irritabilite," (tome i. p. 57, 2me edit.) " C'est Ia respiration qui donne an sang sa chaleur, et it Ia fibre la sus ceptibilitC pour ]'irritation nerveuse," (tome ii. p. 1.) On the other hand, speaking of the mollusca, (tome iii. p. 3,) lI. Cuvier observes of those animals of low respiration, " L'irri tabilitC est extreme dans la plupart." The
same term is, in fact, used in two distinct senses, in these paragraphs.
No further proof can be necessary of the extreme vagueness and incorrectness of the prevailing notions and expressions of physio logists in regard to this subject. All this will appear still more extraordinary, when the law, that the quantity of respiration and the degree of the irritability are, in fact, inverse throughout all the series, stages, and states of animated being, is clearly established.
It is well known that the irritability of the heart and of the muscular fibre in general is greater in the mammalia than in birds, and in reptiles and amphibia than in the mammalia, whether we judge of it by the force and dura tion of the beat of the heart, exposed to the stimulus of the atmospheric air, or by the con tractions of the other parts of the muscular system. Now this is precisely the order of the quantity of respiration in these animals, as ascertained by the pneumatomer, inverted. It is essential, in accurately determining the ques tion of the irritability of the muscular fibre, to compare animals of the same class inter se; birds and the mammalia, reptiles and amphibia, fishes, the mollusca, &c. must be compared with each other, both generically and specifi cally. It is especially necessary to compare the warm-blooded, the cold-blooded, the air breathers, and the water-breathers, in this man ner. however the different classes may differ from each other, there are differences in some of the species of the same class, and especially that of fishes, scarcely less remarkable.
Great differences in the duration of the beat of the heart are observed in fatal, early, and adult states of the higher animals; this dura tion being greater in the first, and least in the last of these conditions. The order of the quantity of respiration is inverse.
The law of the irritability being inversely as the respiration, obtains even in the two sides of the heart itself, in the higher classes of animals. The beat of the heart removed from the body does not cease at the same time in the wall of all its cavities, or of its two sides : buf, as Harvey observes, " primus desinit pulsare sinister ventriculus ; deinde ejus auricula; de mum dexter ventriculus ; ultimo (quod etiam notavit Galenus) reliquis omnibus cessantibus et mortuis, pulsat usque dextra auricula."a Even in this case the irritability is greatest in the part in which the respiration is least.