GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 69 



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 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. Porter 1 has succeeded in keeping even small pieces of the ven- 

 tricle of the mammalian heart alive by maintaining a good circulation of 

 well-oxygenated blood through its vessels (see Section on Nutrition of the 

 Heart). 



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 mus- 

 cular 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 (see p. 74). The relation of special sub- 

 stances in the blood to the needs of the muscle can be best considered 

 together with the chemistry of the muscle (see p. 159). 



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 

 conduction, the portion of the nerve thus separated from the central nervous 

 system sooner or later completely degenerates (see p. 77). 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 a change of excitability. In general it responds 

 more readily to the kathode than the anode, which would imply an increased 

 irritability; at the part near the cut, however, it responds best to the anode. 3 

 The increase is soon followed by a gradual decrease of irritability. In the ease 

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

 or four days, but the nerves of cold-bl led animal 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- 

 diately invade the muscle, which may be considered a proof that nerve and 

 muscle protoplasm are not continuous. 



Though no immediate change in the structure of the muscle is observable, 

 the irritability of the muscle soon begins to alter. At the end of a fortnight the 

 irritability of the muscle for all forms of stimuli is lessened. From this time 

 on, the irritability gradually undergoes a remarkable change, the excitability 



1 Porter: American Journal of Physiology, 1899, vol. ii. p. 127. 

 2 Sczelkow: Sitzungsbcr. d. k. A had, Wim, 1862, Bel. xlv. Abtli. 1. 

 3 Blix: Skandinavischcs Archil' fur Physiologic, 1889, Bd. i, S. 184. 



