GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 75 



this nature. A dog was killed, the body was cut in halves, and the aorta and 

 inferior vena cava were quickly connected with an apparatus for pumping the 

 blood at a regular rate through the hind part of the body. Before the blood 

 entered the arteries it passed through coils in which it was warmed to the nor- 

 mal temperature, and an artificial lung, where it received a supply of oxygen 

 and was relieved of its carbon dioxide. Under these conditions the muscles 

 were kept alive for more than seven hours, and so far retained their normal 

 condition that throughout this period they were able to respond to stimuli 

 sent to them through their nerves and contract with sufficient vigor to raise a 

 considerable weight. H. N. Martin l made a similar experiment on the heart 

 of a dog. The heart and lungs were isolated from the rest of the body, the 

 heart was fed with defibrinated blood from a Mariotte's flask, and the lungs 

 were supplied with air by an artificial respiration apparatus. The heart, which 

 was kept moist and at the normal temperature, continued to beat for four hours 

 and more. 



Normally the blood-supply to the muscle is varied according to its needs. 

 When the muscle is stimulated to action, its blood-vessels are at the same time 

 dilated so that it receives a free supply of blood. 2 Moreover, if muscular work 

 is extensive, the heart beats faster and the respiratory movements are quicker, 

 so that a larger amount of oxygen is provided and the carbon dioxide is 

 removed more rapidly. The importance of the blood-supply to a muscle can 

 be best understood if we consider it in relation to the effects of fatiguing work 

 upon the muscles. The relation of other substances in the blood to the 

 needs of the muscle can be best considered together with the chemistry of 

 the muscle. 



Effect of Separation from the Central Nervous System. If a motor nerve be 

 cut, or if some part of it be so injured that the fibres lose their power of conduc- 

 tion, the portion of the nerve thus separated from the central nervous system 

 sooner or later completely degenerates. Each of the motor nerve-fibres is a 

 branch of a motor cell in the anterior horns of the spinal cord. These nerve- 

 cells are supposed to govern the nutrition of their processes, though how a 

 microscopic cell can thus influence a nerve-fibre a meter or so long is by no 

 means clear. Soon after the nerve is separated from its cell it exhibits an 

 increase of excitability near the point of section, and this change progresses 

 down the fibre toward the periphery. The rule that the change in irritability 

 progresses centrifugally along the motor nerves is known as the Hitter- Valli 

 law. The increase is soon followed by a decrease of irritability. In the case 

 of mammalian nerves loss of irritability may be complete at the end of three 

 or four days, but the nerves of cold-blooded animals may retain their irri- 

 tability for several weeks. The immediate cause of the loss of irritability is 

 the change in the chemical and physiological structure of the axis-cylinder. 

 The degenerative changes result finally in the complete destruction of the 

 nerve-fibres, and involve the motor end-organs as well, but do not imme- 



1 Studies from the Biological Laboratory of Johns Hopkins University, 1882, vol. ii. p. 188. 

 8 Sczelkow: Sitzungsber. d. k. Akad. Wien, 1862, vol. xlv. Abth. 1. 



