444 OF MUSCULAR CONTRACTION. 



for this supply, by a special arrangement of the coronary arteries.* That the 

 heart's action comes to an end much sooner, after the destruction of animal 

 life by pithing, when the coronary arteries have been tied, than when they 

 are left untouched, has been proved by the experiments of Mr. Erichsen.t 

 In an animal that has been pithed, but whose heart has been left intact, artificial 

 respiration will easily keep up its action for an hour, or an hour and a half. 

 But when the coronary arteries were tied, a mean of six experiments gave a 

 duration, for the ventricular action, of only 23d minutes after the ligatures 

 were applied, and 32^- minutes after the pithing; and in no instance was it 

 prolonged more than 31 minutes after the application of the ligature, or 37 

 minutes after the pithing. On the other hand, when the aorta was tied, so 

 that the coronary arteries were distended with blood, the circulation being 

 carried on through them alone, the right ventricle continued to act up to the 

 82d minute. 



585. There is a remarkable difference in the degree of Irritability in the two 

 sides of the heart, to which Dr. M. Hall has directed attention. In the warm- 

 blooded Vertebrata, the right side of the heart will act on the stimulus of ven- 

 ous blood ; whilst the left side requires the stimulus of arterial. In Fishes, 

 on the other hand, whose heart corresponds to the right side only of that of 

 Man, the whole is put in action by venous blood. In Reptiles, one auricle is 

 sufficiently stimulated by venous blood, whilst the other requires arterial ; and 

 the ventricle is excited to action by a mixed fluid. In all these cases, there 

 must be a marked difference in the properties of the several parts ; some being 

 sufficiently affected by a stimulus, which is totally inoperative on others. 

 This is still more remarkably exemplified by the fact, that the muscular fibre 

 of Frogs would be thrown into a state of permanent and rigid contraction 

 (through the powerful operation of its property of Tonicity), by the stimulus 

 of a fluid no hotter than the blood, which ordinarily bathes the muscles of 

 Birds. Now in those warm-blooded animals which pass the winter in a state 

 of torpidity, the respiration is very slow and imperfect, and the blood is very 

 imperfectly arterialized. There must, therefore, be a change in the properties 

 of the left ventricle, by which it becomes capable of action on a more feeble 

 stimulus, thus resembling the ventricle of Reptiles. 



a. This change Dr. M. Hall designates as an increase of Irritability; considering that, 

 if muscular action be excited by a more feeble stimulus, the property to which that action is 

 due, must be itself more exalted. Physiologists have been so long accustomed, however, to 

 consider the irritability of the muscles in warm-blooded animals as greater than that of cold- 

 blooded, on account of the greater energy and rapidity of their contractions when excited, 

 that it seems undesirable to modify the term in the manner proposed by Dr. Hall. No one 

 will assert that the vitality of the Muscle is exalted, when it is reduced to the condition of that 

 of the Reptile; and, as Irritability is strictly a vital property, it cannot be correctly spoken of 

 in that manner. The general principle, however, laid down by Dr. M. Hall, that the facility 

 with which the muscular system may be excited to contraction, or in other words the feeble- 

 ness of the stimulus required for the purpose, is inversely as the respiration of the animal, 

 is, no doubt, generally correct. 



586. The doctrine, now generally accepted as a Physiological truth, that the 

 active exercise of the Contractility of Muscle, is attended with a waste or dis- 

 integration of its tissue, rests upon a great variety of evidence. The increase 

 of the demand for food, occasioned by Muscular activity ( 263), is an indica- 

 tion that the nutritive operations are excited by it ; and the purpose of these 

 can scarcely be anything else, than the reparation of the loss which the Mus- 

 cle has sustained. Again, it has been just shown, that the presence of Oxygen 

 is essential to the development of the Contractile force ; and there is evidence 

 that, in this development, a chemical change is effected in the substance of the 



* Dr. M. Hall's Gulstonian Lectures, pp. 23, 24. f Medical Gazette, July 8, 1842. 



