IRRITABILITY. 



tarum intestina sunt, quse et evulsa pergunt se 

 contrahere et fngida demum ; etiam his tena- 

 cius cor, si omnia conputaveris, in pullo etiam 

 evidentissime et in fhgidis 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 are as follow : 

 " 1. La contractilite du ventricule aortique 

 tait eteinte 49 minutes apres la mort; 



" 2. L'aorte n'a oflert aucun mouvement de 

 contraction ; 



" 3. Cinquante-six minutes apres la mort, 

 la contractilite de I'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 ventricule pulmonaire perdit sa con- 

 tactilite 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 1'appareil locomoteur perdirent succes- 

 sivement leur contractilite a mesure que le 

 contact de 1'air agissait sur eux ; mais ceux qui 

 ne furent exposes a 1'air que tard, par exemple 

 au bout d'environ 4 heures, ne cesserent de se 

 mouvoir que 4 heures 15 minutes apres la mort ; 

 " 6. Les oreilletes du cceur, qui etaient 

 exposees a 1'air depuis le commencement de 

 1'experience, ne cesserent de se contracter que 

 4 heures 40 minutes apres la mort."f 



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 



* Haller, Prims Lines, 1767, p. 207. 



t Recherches de Physiologie, 1811, 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 



