III. As to the conditions, necessary to the maintenance of the contractile powers of living parts, it is in the first place obvious, that they are always dependent on the maintenance of the organization of these parts themselves. When the muscles waste, as from rheumatic inflammation, or from the poison of lead, as in colica pictonum, or when their texture is gra dually altered, as by inflammation or in cer tain organic diseases occasionally affecting them, or more rapidly relaxed and injured by over-distension, they lose their contractile power more or less completely; and their power is likewise gradually diminished in old age, as their texture partakes of the gradually increasing rigidity.
Like all other vital actions, the contractions of moving parts are more immediately depen dent on the maintenance of a certain tempe rature, varying in the different tribes of ani mals,—in all the warm-blooded (in the state of' activity) probably confined within the de grees of 60 and 120 of Fahrenheit. They are dependent also on the regular supply of arterial Blood. The experiments of Stenon and others have shewn, that the power of muscles is rapidly extinguished when the ar teries supplying them are tied. It has gene rally been supposed, since the time of Bichat, that venous blood, when it penetrates muscular fibres, is equally or even more rapidly noxious to them, than the denial of the supply of arterial blood ; but the experiments of Dr.Kay* have shewn, that the contractile power of mus cles, when failing from this latter cause, may be restored by the influx of venous blood, al though in a less degree than by arterial,— and Dr. Marshall Hall has observed, that in hybernating animals whose respiration is sus pended, the flow of venous blood through all the textures continues, and keeps up a certain degree of muscular power ; so that the venous blood can only be regarded as less powerful in maintaining the irritability of muscles than arterial blood (probably because it is incapable of affording them nourishment), not as posi tively deleterious to them. The act of healthy Nutrition, by arterial blood, is therefore the main condition of the vital power of muscles, as of all other living solids. And it is im portant to remember that this vivifying in fluence of the living blood on the solids is evidently reciprocal ; for when any of the vessels containing blood lose their vitality, as from injury, the blood then coagulates, as if drawn from the body.
There is a remarkable difference which has been long observed, in the different classes of animals, and even in the different states of the same animals, as to the consumption of oxygen by the blood on one hand, and the indications of muscular power on the other. The activity
of muscular power (as indicated by the rapi dity of the circulation and the energy of vo luntary muscular exertions) appears to be, in general, in direct proportion to the amount of action between the air and the blood, being greatest in birds, greater in the mammalia than in reptiles or fishes; and greater in insects, where air is freely admitted into the interior of the body, and applied to the blood, than in the Zoophyta, or even the Mollusca, where there is less exposure of the blood to the air ; and again, being greater in perfect animals than in eggs or pupw, and greater in animals in a state of activity 'than in those in a state of torpor or hybernation. But on the other hand, the endurance of the muscular power, or tenacity of life, in whatever manner the vital principle is depressed or extinguished, is genemlly in the inverse ratio of the activity of muscular contractions, and of the amount of mutual action between the air and the blood. Thus the tenacity of life in reptiles and fishes is well known to be greater than in mammalia or birds,—in some of the lower classes, par ticularly the infusory animalcules, much greater than in any of the higher ; in very young ani mals greater than in adults ; and in hyberna ting animals, in eggs, and pupw, greater than in any perfect animals.
Dr. Marshall Hall has observed, that in some of the lower classes of animals, such as Reptiles, the degree of muscular contraction induced by stimuli, as well as its duration, is greater than in the warm-blooded animals ; and he has hence been led to lay down as a general principle the reverse of what has com monly been stated, viz. that the Irritability of muscular fibres is inversely as the quantity of Respiration. But this proposition seems to be too generally, if not incorrectly, expressed. It seems an unnecessary innovation in language, to assert that the irritability of muscular fibres is inversely as the activity of muscular con tractions, or that the irritability in insects, where the blood is fully exposed to the air, is less than in the Zoophyta, where there is much less provision for respiration. In fact, the vital powers of contractile parts vary so much in different organs, even of the same animal, that it may be doubted whether any other general proposition can be laid down as to its connexion with respiration, than that of the greater activity of muscular action, on the whole, in those animals where there is much exposure of the blood to the air, and the greater endurance or tenacity of life where there is little.