ENERGY AND RAPIDITY OF MUSCULAR CONTRACTION. 291 



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

 ficial 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 

 give a duration, for the ventricular action, of only 23 minutes after the liga- 

 tures were applied, and 32 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. 



393. 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 venous 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 mus- 

 cular 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. This change 

 Dr. M. Hall designates as an increase of Irritability ; considering that, if mus- 

 cular 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 whic.h the muscular system may 

 be excited to contraction, or, in other words, the feebleness of the stimulus 

 required for the purpose, is inversely as the respiration of the animal, is, no 

 doubt, generally correct. 



394. A curious question has been lately raised, the decision on which is of 

 some importance in our determination of the nature of the force by which the 

 contraction of muscles is occasioned. This isy whether the power of a muscle 

 is greater or less at different degrees of contraction, the same stimulus being 

 applied. This seems to have been determined by the ingeniously-devised 

 experiments of Schwann.* He contrived an apparatus, which should accu- 

 rately measure the length of the muscle, and at the same time, the weight 

 which it would balance by its contraction. Having caused the muscle of a 

 frog to shorten to its extreme point, by the stimulus of galvanism applied to 

 the nerve, so that no further stimulation could lift a weight placed in the 



* MQller's Physiology, p. 903. 



