DEPENDENCE OF MUSCULAR CONTRACTION UPON OXYGEN. 211 



rupted, that organ will lose its contractility earlier than usual. Thus 

 it has been shown by Mr. Erichsen, that, if the coronary arteries (sup- 

 plying the substance of the heart) be tied in a dog or rabbit, after the 

 animal has been pithed, and the circulation is being maintained by arti- 

 ficial respiration, the pulsation of the heart will only go on for about 23 

 minutes after the ligature has been applied, or about 33 minutes after 

 the death of the animal ; instead of continuing for 90 minutes, which 

 it will do under other circumstances. Further, if blood charged with 

 carbonic acid, instead of with oxygen, circulate through the muscles, 

 their irritability is speedily impaired, and is even destroyed. This is 

 best seen, when animals are killed by being caused to breathe an atmo- 

 sphere highly charged with carbonic acid; the irritability of their 

 muscles departing as soon as they are dead. In fact, the destruction 

 of the irritability of the heart, by the circulation of venous blood through 

 its substance, is one of the immediate causes of death. A similar effect 

 is produced by the respiration of other gases, which are either poisonous 

 in themselves, or which prevent the interchange of carbonic acid and 

 oxygen, which ought to take place in the lungs. On the other hand, 

 when animals have been made to respire oxygen, and their blood has 

 been consequently highly arterialized, the contractility of their muscles 

 is retained for a longer time than usual. 



360. Hence we may conclude the presence of oxygen in the blood to 

 be one of the conditions of muscular contraction ; although it is much 

 less essential in the case of cold-blooded, than in that of warm-blooded 

 animals. It is interesting to remark, that the muscles of hylernating 

 warm-blooded Mammals are reduced for a time to the level of those of 

 cold-blooded animals ; their contractility being retained almost as long 

 as that of the latter ; thus confirming the general principle already 

 stated, as to the relation between the amount of respiration, and the 

 duration of the irritability. 



361. The Muscles, as we have seen, are largely supplied with blood ; 

 and the flow of blood into them increases with the use that is made of 

 them. The demand for nutrition is obviously augmented, in proportion 

 to the activity of the exercise of the Muscular system ; for the slightest 

 observation suffices to show, that a much smaller amount of nourishment 

 is sufficient to sustain the body in its normal condition, when the Mus- 

 cular system is not actively exercised, than when it is in energetic 

 operation. The quantity which is ample for an individual leading an 

 inactive life, is far too little for the same person ; in the full exercise of 

 his muscular power. Again, there is evidence derived from observation 

 of. the relative amount of the solid matters excreted from the body 

 under different circumstances ( 731), that a waste or disintegration of 

 the muscular tissue takes place, whenever it is actively employed ; and 

 this in a degree strictly proportional to the amount of force which it is 

 called upon to exercise. In fact, it would appear that this waste is a 

 necessary consequence of the exercise of the muscle ; every act of con- 

 traction involving the death and decomposition of a certain amount of 

 tissue. And as the presence of oxygen is always necessary for the 

 decomposition of organic substances, so do we find that the penetration 

 of the muscular tissue by oxygenated blood is essential to the manifes- 



