IRRITABILITY, AND MUSCULAR POWER. 105 



other part of an organ has lost its vital power, and is the only cause of its 

 not becoming corrupt. Fourthly, that though not vascular itself, it is capable, 

 by its own energy, of producing new vessels out of its own substance, and 

 vessels of every description, as lymphatics, arteries, veins, and even nerves.* 

 Finally, he proved, that the blood, when in a state of health, is not only, like 

 the muscular fibre, capable of contracting upon the application of a certain 

 degree of appropriate stimulus, but that, like the muscular fibre also, it is 

 instantly exhausted of its vital power whenever such stimulus is excessive; 

 and that the same stroke of lightning that destroys the muscular fibre, and 

 leaves it flaccid and uncontracted, destroys the blood, and leaves it loose and 

 uncoagulated. 



Important, however, as these facts are, they do not reach home to the 

 question before us. They sufficiently establish the blood to be alive, but they 

 do not tell us what it is that makes it alive : on the contrary, they rather drive 

 us into a pursuit after some foreign and superadded principle ; for that which 

 is at one time alive, and at another time dead, cannot be life itself. 



The next theory, therefore, to which I have adverted, undertakes to explain 

 in what this foreign and superadded principle consists. SOME EXQUISITELY 

 SUBTLE GAS or AURA some fine, elastic, invisible fluid, sublimed by nature in 

 the deepest and most unapproachable recesses of her laboratory, and spirited 

 with the most active of her energies. An approach towards this hypothesis 

 is also of great antiquity ; for it constituted one of the leading features of 

 the Epicurean philosophy, and is curiously developed by Lucretius in his poem 

 on the Nature of Things. According to him, it is a gas or aura, for which in 

 his day there was no name, diffused through every part of the living fabric, 

 swifter and more attenuate than heat, air, or vapour, with all which it con- 

 curs in forming the soul or mind as its chief elementary principle : 



Far from all vision this profoundly lurks, 

 Through the whole system's utmost depth diffus'd, 

 And lives as soul of e'en the soul itself. | 



But it is to the astonishing discoveries of modern chemistry alone that we 

 are indebted for any fair application of any such fluid to account for the 

 phenomena of life. 



Among the numerous gases which modern chemistry has detected, there 

 are three which are pre-eminently entitled to our attention, though they seem 

 to have been glanced at by the Epicureans : caloric, or the matter of heat, 

 chiefly characterized in our own day as a distinct substance, by the labours 

 of Dr. Black and Dr. Crawford ; oxygen, or the vital part of atmospheric 

 air, first discovered by Priestly, and explained by Lavoisier ; and the fluid 

 which is collected by the Voltaic trough, and which is probably nothing more 

 than the electric fluid under a peculiar form. 



Of these, caloric, as a distinct entity, was detected first. It was found to 

 be a gas of most astonishing energy and activity, and, at the same time, to 

 be of the utmost consequence to the living substance ; to exist manifestly 

 wherever life exists, and to disappear on its cessation. It was hence con 

 ceived to be the principle of life itself. 



But oxygen began now to start into notice, and the curious and indispen- 

 sable part it performs in the respiration, as well as in various other functions 

 of both animal and vegetable existence, to be minutely explored and ascer- 

 tained, and especially by the microscopic eye of M. Girtanner.J The genius 

 of Crawford fell prostrate before that of Lavoisier. Oxygen was now 

 regarded as the principle of life, and heat as its mere attendant or handmaid. 



About the year 1790, Professor Galvani, of Bologna, accidentally discovered 



* Dr. Munro has proved, that the limb of a frog can live and be nourished, and its wounds heal, without 

 any nerve. 



t Nam penitus prorsum latet haec natura, subestque ; 

 Nee magis hac infra quidquam est in corpore nostro ; 

 Atque anima est animae proporro totius ipsa. 



De Her. Nat. iii. 274. 

 $ Memoires BUT I'Irritabilite, consideree comme principe de vie dana la nature organist* Paris, 1730 



