CONTRACTILITY. 



723 





nerves, when experiment shews that nerves 

 cannot excite these movements." 



IV. In regard to the laws, by which the 

 vital powers of contractile parts may be regu- 

 lated, we have probably much to learn ; but 

 three sets of facts have been observed, which 

 may at present be regarded as general laws in 

 this department of physiology. 



1. Notwithstanding what has been said of 

 the contractility of muscles being independent 

 of any influence continually flowing to them 

 from the brain or spinal cord, it is well ascer- 

 tained that in a living and entire animal, where 

 all the functions of the body are, for wise and 

 important purposes, made liable to change, 

 from changes in the nervous system, the con- 

 tractile power of various moving parts is sub- 

 ject to increase or diminution from physical 

 causes acting in these larger masses of the ner- 

 vous system, just as they are from various acts 

 and affections of Mind, the effects of which 

 may be said to be imitated by those physical 

 causes. Thus in the experiments of Le Gallois, 

 of Dr. Wilson Philip, of Flourens, and others, 

 suddenly crushing any large portion, either of 

 the brain or spinal cord, has been found uni- 

 formly to depress or even extinguish the power 

 of the heart ; the well-known fatal effect often ob- 

 served from sudden violent injury of the epi- 

 gastrium in the human body, has been ascribed 

 with probability to the injury of the great semi- 

 lunar ganglion ; and the depression of the 

 heart's action which attends Concussion, and 

 which is the immediate cause of death in the 

 most quickly fatal cases of that kind, is also 

 generally regarded as an impression, made ori- 

 ginally on the nervous system, and immediately 

 transmitted to the heart. On the other hand, 

 slighter and more continued physical irritations 

 of the nervous system appeared in many expe- 

 riments, especially of Dr. Wilson Philip, to 

 augment the irritability of the heart. It is true 

 that, in all these cases, some have supposed the 

 effects of the violence to be on the organs of 

 circulation directly, and not through the inter- 

 vention of the nerves ; but when it is remem- 

 bered, that some of those injuries, which are 

 the most rapidly fatal to the heart's actions, 

 (such as the pushing of a probe along the 

 spinal canal,) do not necessarily imply any 

 great violence to the body at large ; and further, 

 that precisely similar effects on the heart's ac- 

 tion (both increase and diminution) often result 

 from mental emotions and passions, which cer- 

 tainly act first on the nervous system, the ac- 

 count which we give of the mode of action of 

 these causes appears to be sufficiently con- 

 firmed. 



One cause, acting primarily on the nervous 

 system, which seems to have a peculiar de- 

 pressing effect on the heart's action, is, sudden re- 

 moval of the pressure to which the brain had 

 previously been subjected. The effect of this 

 on the heart has been repeatedly seen in surgi- 

 cal operations ; and this seems to be an essen- 

 tial part of the pathology of several cases of 

 syncope, particularly of that which results, 

 either from bloodletting in the erect posture, 

 or from tapping in ascites. 



It is very remarkable that the heart, which 

 is so strictly an involuntary muscle, and so 

 little liable to excitation by stimuli applied 

 to its nerves, is much more liable than the 

 voluntary muscles both to sudden increase 

 and to diminution, or even total loss, of vital 

 power from such causes as we have now 

 considered. But a little reflection will shew, 

 that the direct stimulation of a muscle, and the 

 increase or diminution of its irritability, are 

 perfectly distinct cases. And we may approxi- 

 mate, at least, to an explanation of the peculiar 

 liability of the heart (and probably of other 

 involuntary muscles) to the influence of such 

 causes acting through the nervous system, as 

 augment or depress the vital power, when we 

 remember two facts : 1. that the causes which 

 act in this way are very generally such as are 

 applied to large portions of the brain or spinal 

 cord ;* and 2. that the arrangements of the 

 ganglionic nerves are such as to place the heart 

 and other organs supplied from the ganglia, in 

 connexion with the whole extent of the cerebro- 

 spinal axis, and hardly with any individual part 

 of it more than another. 



2. There are various external agents, by the 

 application of which the vital power of con- 

 tractile parts, and especially of the heart, the 

 main agent in the circulation, may be altered 

 or even destroyed. It is increased, not only 

 by moderate increase of the Temperature in 

 which living parts are kept, and of the quantity 

 of arterial blood sent to them, but also by Elec- 

 tricity applied in a low degree of intensity, and 

 by various articles of diet and medicinal agents, 

 such as the various preparations of Alcohol; 

 and it is diminished, or even suddenly extin- 

 guished often by the same agents applied in 

 excess, (as in the case of Lightning when most 

 rapidly fatal,) and still more remarkably by 

 certain Poisons, such as the upas antiar, tobacco, 

 digitalis, arsenic, and hydrocyanic acid. It is 

 still doubtful through what medium these poi- 

 sons act on the vital power of the heart ; but it 

 is certain that the effect which they produce on 

 that power is the immediate cause of the death 

 resulting from them.f 



In cases of the most sudden death produced 

 by such causes acting in the utmost inten- 

 sity, the contractile power in the voluntary 

 muscles, as well as in the heart, has been found 

 to be very much diminished or even nearly ex- 

 tinguished; and it is very important to observe, 

 that in such cases the property of coagulation 

 in the blood is likewise lost; which seems 

 clearly to indicate (what various other facts 

 confirm) that this change in the blood is de- 

 pendent on the existence in that fluid of a 

 certain degree of the same vital properties, to 

 which we give the name of Contractility as ex- 

 isting in the solids. 



3. The contractile power of living parts is 

 liable to much alteration from the degree in 



* See Dr. Wilson Philip's Experimental Inqui- 

 ries, &c. ch. ii. and iv. 



t The terms Stimulant and Sedative are applied 

 most correctly to those agents which thus exalt or 

 depress the vital actions of the circulating system. 



