CONTRACTILITY. 



721 



to the definition of Tonicity, not of Irritability. 

 This last phenomenon, as it disappears before 

 putrefaction begins, and as it is variously in- 

 fluenced by causes affecting vital action, is 

 allowed to be a last exertion of vital power. 



There are evidently slighter modifications or 

 varieties of the powers which we have thus 

 distinguished ; but the distinctions now stated 

 seem to be those which are sufficiently marked 

 to demand separate names. Besides the mus- 

 cular texture, some of the membranes, espe- 

 cially the skin, appear to be endowed with a 

 certain degree of vital contractile power, al- 

 though not with true Irritability. It is remark- 

 able, that the greatest degree of contraction 

 seen in muscular fibres, is in those which pos- 

 sess the property of simple Contractility rather 

 than Irritability, viz. in the bladder and uterus 

 more than in the intestines, and in these more 

 than the heart. 



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 decree 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 



* Edin. Mcd. and Surg. Journal, vol. xxviii. 

 and Treatise on s^hyxia, ch. iii. 



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 pupae, 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 generally 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 pupae, 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 



