GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 73 



tion known as tetanus, during which it shortens considerably more than it does 

 when making single contractions. Increase in irritability plays only a com- 

 paratively small part in the production of this remarkable phenomenon, which 

 we shall study more carefully when we come to the mechanical problems 

 involved in muscular contractions. 



Rapidly repeated stimuli, though at first favorable to activity of a muscle, 

 soon exert an unfavorable influence by causing the lessened irritability which 

 is associated with fatigue. 



(6) Influences which favor the Maintenance of the Normal Physiological 

 Condition of Nerve and Muscle. Effect of Blood-supply on Nerve and Muscle. 

 The vascular system is a path of communication between the several organs 

 and tissues, and the circulating blood is a medium of exchange. The blood 

 carries nutritive materials from the digestive organs and oxygen from the 

 lungs to all the tissues of the body, and it transports the waste materials which 

 the cells give off to the excretory organs. In addition to these functions it 

 has the power to neutralize the acids which are produced by the cells during 

 action, and so maintain the alkalinity essential to the life of the cell ; it sup- 

 plies all parts with moisture ; by virtue of the salts which it contains, it secures 

 the imbibition relations which are necessary to the preservation of the normal 

 chemical constitution of the cell-protoplasm ; it distributes the heat, and so 

 equalizes the temperature of the body ; finally, in addition to these and other 

 similar functions, it is itself the seat of important chemical changes, in which 

 the living cells which it contains play an active part. It is not strange that 

 such a fluid should exert a marked influence upon the irritability of the nerves 

 and muscles. Since the metabolism of muscles is best understood, we will 

 first consider the importance of the circulation to the muscle. Muscles, even 

 in the so-called state of rest, are the seat of chemical changes by which energy 

 is liberated, and when they are active these changes may be very extensive. 

 If the cell is to continue its work, it must be at all times in receipt of mate- 

 rials to replenish the continually lessening store of energy-holding compounds; 

 moreover, as the setting free of energy is largely a process of oxidation, a free 

 supply of oxygen is likewise indispensable to action. These oxidation pro- 

 cesses result in the formation of waste products such as carbon dioxide, water, 

 lactic acid and these are injurious to the muscle protoplasm, and if allowed 

 to accumulate would finally kill it. Of the services which the blood renders 

 to the muscle there are, therefore, two of paramount importance, viz. the 

 bringing of nutriment and oxygen and the removal of waste matter and sur- 

 plus energy. 



A classical experiment illustrating the effect of depriving tissues of blood 

 is that of Stenson, which consists in the closure of the abdominal aorta of a 

 warm-blooded animal by a ligature, or by compression. In the case of a 

 rabbit, for example, the blood is shut off, not only from the limbs but from the 

 lower part of the spinal cord. The effect is soon manifested in a complete 

 paralysis of the lower extremities, sensation as well as power of voluntary and 

 reflex movements being lost. The paralysis is due, in the first instance, to the 



