MUSCULAR IRRITABILITY. 823 



to contract together, and then shortly relax. But this is probably no less 

 true of the individual fibres of those muscles which are kept in contraction 

 by a stimulus transmitted through their nerves ; since none of them appear, 

 under ordinary circumstances at least, to remain in a contracted state for 

 any length of time, a constant interchange of condition taking place among 

 the fibres, some contracting while others are relaxing, and vice versa. It is 

 difficult to speak with confidence, however, in regard to the condition of the 

 individual fibres of a muscle that is thrown into a state of continued 

 spasmodic contraction. Whether the individual fibres in such instances 

 maintain a state of contraction without intermission, or whether the contrac- 

 tion of the entire muscle is kept up by a continual interchange of the fibres 

 actually engaged, is a very curious subject for inquiry. 



672. Muscles do not lose their Irritability immediately on the general 

 death of the system, which must be considered as taking place when the cir- 

 culation ceases without the power of renewal ; in cold-blooded animals it is 

 retained much longer after this period than in the higher Vertebrata, in 

 some instances attaining its highest degree long after death has occurred, 1 

 whilst in the latter it frequently disappears within an hour. The muscles 

 of young animals generally retain their irritability for a longer time than 

 those of adults ; on the other hand, those of Birds lose their irritability 

 sooner than those of Mammalia. Hence, as a general rule, the duration of 

 the irritability is inversely as the amount of respiration. From experiments 

 on the bodies of executed criminals who were previously in good health, 

 Nysten ascertained that in the human subject the irritability of the several 

 muscular structures disappears in the following time and order : The left 

 ventricle of the heart first, the intestinal canal at the end of 45 or 55 min- 

 utes, the urinary bladder nearly at the same time, the right ventricle after 

 the lapse of an hour; the oasophagus at the expiration of an hour and a half, 

 the iris a quarter of an hour later ; the muscles of animal life somewhat 

 later ; and lastly, the auricles of the heart, especially the right, which in one 

 instance contracted under the influence of galvanism 16i hours after death. 

 It will be presently shown that the departure of the irritability is essentially 

 dependent upon the cessation of the circulation, and that it may be pre- 

 vented from disappearing, and may even be recalled after it has ceased to 

 manifest itself, by transmitting a current of arterial blood through the 

 muscles. Krouecker 2 has endeavored to establish certain laws of muscular 

 exhaustion to this effect: 1. That if a muscle be overloaded with a definite 

 weight, and irritated at equal intervals by equal induction shocks, the 

 height of the contractions form an arithmetical series, in which the constant 

 difference depends only on the interval of time. 2. The diminution in the 

 height of the contractions is independent of the weight, and only depends on 

 the interval between two contractions. 3 



673. We find, however, that sudden and severe injuries of the Nervous 

 centres have power to impair, directly and instantaneously, or even to 

 destroy the contractility of the whole Muscular system; so that death imme- 

 diately results, and no irritability subsequently remains. It is in this man- 

 ner that the sudden destruction of the brain and spinal cord, especially of 



1 See on the whole subject of the Irritability of Muscles, a good paper by Dr. 

 ^Norris in Humphry and Turner's Journ. of Anat., vol. i, p. 217. See also B. W. 

 Richardson's Croonian Lecture on Muscular Irritability after Systemic Death, Abst. 

 in Lond. Mod. Record, June 18th, 1873. 



2 See abstract of his paper in the Journal of Anatomy and Physiology, vol. vi, 



3 See in regard to the irritability of muscles after systemic death, B. W. Richard- 

 son, Proceed. Roy. Soc., 339, 1874. 



