LOSS OF IRRITABILITY BY AFFECTIONS OF THE NERVOUS SYSTEM. 441 



tained much longer after this period than in the higher Vertebrata, in some 

 of which it 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 Mam- 

 malia. 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.Humnn subject, the irritability of the several muscular structures departs 

 in the following time and order. The left ventricle of the heart first; the in- 

 testinal canal at the end of 45 or 55 minutes ; the urinary bladder nearly at 

 the same time ; the right ventricle after the lapse of an hour ; the oesophagus 

 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 influ- 

 ence of galvanism 16j hours after death. 



579. Muscular Irritability is deadened by many substances, especially by 

 those which have a narcotic or sedative action on the Nervous system. In 

 carbonic acid gas, hydrogen, carbonic oxide, or sulphurous acid gas, muscles 

 contract very feebly, or not at all, when stimulated ; whilst in oxygen they 

 retain their irritability longer than usual. Narcotic substances, such as a watery 

 solution of opium, when applied directly to the muscles, have an immediate 

 and powerful effect in diminishing or even destroying their irritability ; this 

 effect is also produced, though in a less powerful degree, by injecting these 

 substances into the blood. In the same manner, venous blood, charged with 

 carbonic acid, and deficient in oxygen, has the effect of a poison upon 

 muscles ; diminishing their irritability, when it continues to circulate through 

 them, to such a degree, that they sometimes lose it almost as soon as the cir- 

 culation ceases, as is seen in those who have died from gradual and therefore 

 prolonged Asphyxia. The unfavourable influence of venous blood is also 

 shown in the Morbus Creruleus ; patients affected with which are incapable 

 of any considerable muscular exertion. Although most of the stimuli which 

 occasion the contraction of muscles, when directly applied to their fibres, ope- 

 rate also when applied to their motor nerves, the same does not hold good in 

 regard to those agents which diminish irritability. It is a fact of some im- 

 portance, in relation to the disputed question of the connection of muscular 

 irritability with the nervous system, that when, by the application of narcotic 

 substances to the Nerves, their vital properties are destroyed, the irritability 

 of the Muscle may remain for some time longer, showing that the latter 

 must be independent of the former. 



580. 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 immediately 

 results, and no irritability subsequently remains. It is in this manner, that 

 the sudden destruction of the Brain and Spinal Cord, especially of the latter, 

 occasions the immediate cessation of the Heart's action ; though they may be 

 gradually removed, without any considerable effect upon it. Severe concus- 

 sion has the same effect; hence the Syncope which immediately displays it- 

 self. It is sometimes an important question in Forensic Medicine, whether 

 an individual, who has died from the effects of a blow upon the head, could 

 have moved from the place where the blow was inflicted. If there be found, 

 as is frequently the case, no sensible disorganization of the Brain, the death 

 must be attributed to the concussion, and must have been in that case imme- 

 diate. If, on the other hand, effusion of blood has taken place within the 

 cranium, to any considerable extent, it is probable that the first effects of the 

 blow were in some degree recovered from, and that the circulation was re- 



