The application of these new laws to phy siology—the first application of the vis nervosa to physiology—is very extensive, co-extensive indeed with all the acts of ingestion and egestion in the animal ceconomy. But it does not belong to our present article to treat of this important and extensive subject. We now return to that of irritability in general.
The degree of irritability is not the same in every organ of the body. Haller and Nysten have investigated this subject, and the follow ing are their statements respectively : Haller observes, " Tenacissima virium insi tarum intestina sunt, gum et evulsa pergunt se contrahere et frigida demum; etiam his tena cius cor, si omnia conputaveris, in polio edam. 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 contractilite du ventricule aortique etait eteinte 49 minutes apres la mort ; " 2. L'aorte n'a offert aucun mouvement de contraction; " 3. Cinquante-six minutes apres la wort, la contractilite de l'estomac, des intestins et de la vessie etait eteinte ; mais ces organes n'ont pu etre soumis assez promptement au galvan isme pour connaitre la duree relative de leur force contractile ; " 4. Le ventriculepulmonaire perdit sa con tactilite une heure 58 minutes apres la mort ; " 5. Deux heures 2 minutes apres la wort, le diaphragme ne se contractait plus ; les mus cles de l'appareil locomoteur perdirent succes sivement leur contractilite a mesure que le contact de l'air agissait sur eux ; mais cenx qui ne furent exposes a l'air que tard, par exemple an bout d'environ 4 heures, ne cesserent de se mouvoir que 4 heures 15 minutes apres la mint; " 6. Les oreilletes du cceur, qui etaient exposees a l'air depuis le commencement de l'experience, ne cesserent de se contracter que 4 heures 40 minutes apres la mort."t 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 destrpctive 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 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 fcetus, 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.