66 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



excitability. Slight stretching heightens the excitability and even quite vigor- 

 ous stretching has only a temporary depressing effect unless it be carried to 

 the point of doing positive injury to the axis-cylinder, and of causing degen- 

 eration. As nerves have the power to regenerate they may recover from even 

 such an injury. 



The irritability of muscles is likewise increased by moderate stretching and 

 destroyed if it be excessive. Thus slight stretching produced by a weight 

 causes a muscle to respond more vigorously to irritants. Similarly tension of 

 the muscles of the leg, produced by slight over-flexion or extension, makes 

 them more irritable to reflex stimuli, as in the case of the knee-jerk and ankle- 

 clonus. Tension must be very marked to permanently alter the irritability of 

 the muscles. 



Effect of Temperature. Changes in temperature, if sudden and extreme, 

 irritate nerves and muscles. If the nerve or muscle be quickly frozen or 

 plunged into a hot fluid it will be excited and the muscle be seen to contract. 

 The cause of the irritation has been attributed to mechanical or chemical 

 alterations produced by the change of temperature. The ulnar nerve at the 

 elbow is excited if the part be dipped into ice-water and allowed to remain 

 there until the cold has had time to penetrate ; as is proved by the fact that in 

 addition to the sensations from the skin, pain is felt which is attributed by the 

 subject of the experiment to the region supplied by the nerve. As the effect 

 of the cold becomes greater the pain is replaced by numbness, both the irrita- 

 bility and power of conduction of the nerve being reduced. Gradual cooling 

 of motor nerves or muscles, and gradual heating, even to the point of death 

 of the tissue, fails to excite contractions. It is stated that if a frog whose 

 brain has been destroyed is placed in a bath the temperature of which is very 

 gradually increased, the heating may be carried so far as to boil the frog without 

 active movements having been called out. If a muscle be heated to 45 C. 

 for frogs and 50 C. for mammals, it undergoes a chemical change, which is 

 accompanied by a form of shortening different from the contraction induced by 

 irritants. This form of contraction, though extensive, is feeble and is asso- 

 ciated with a stiffening of the muscle, known as rigor oaloris. 



In general it may be said that raising the temperature above the usual tem- 

 perature of the animal increases, while cooling decreases the irritability of the 

 nerves and muscles. Cold, unless excessive and long continued, though it 

 temporarily suspends does not destroy the irritability, while heat, if at all great, 

 so alters the chemical constitution of the cell-protoplasm as to destroy its life. 



The higher the temperature, the more rapid the chemical changes of the 

 body and the less its power of resistance ; low temperature, on the other hand, 

 slows chemical processes and increases the endurance. It is noticeable that 

 nerves and muscles remain irritable much longer than ordinarily in case the 

 body be cooled before their removal. In the case of a mammal, the irritability 

 may last from six to eight hours instead of two and a half, while in the case 

 of frogs it may be preserved at for ten days, although at summer heat it lasts 

 only twenty-four hours. In the case of frogs which have been kept at a low 



