316 PRACTICAL PHYSIOLOGY 



portions of this muscle, which consists of fibres running in a direction 

 parallel with its length. 



The sartorius muscle is dissected from the other thigh and th 

 nerveless parts are stimulated by a pinch with a pair of forceps or by 

 an electrical shock ; they contract, the muscle possesses independent 

 excitability. 



The absence of nerves from the terminal portions can also be shown 

 in the following way. The muscle is suspended from its tibial end and 

 is lowered until the cut iliac end touches some strong glycerine con- 

 tained in a watch glass ; it does not contract. A thin transverse slice 

 is cut away and the muscle is again lowered into contact with the 

 glycerine ; there is still no contraction. This procedure is repeated 

 until the nerves are cut across and on contact with the glycerine are 

 stimulated and make the muscle pass into a contracted condition. 



CHAPTER XII. 



THE EFFECT OF TEMPERATURE UPON THE EXCITABILITY AND 

 CONDUCTIVITY OF NERVE. THE MOIST CHAMBER. 



LIVING structures contain about 80 per cent, of water, and upon die 

 presence of this water largely depend many of the physical and 

 chemical changes which underlie the phenomena of life. It is impor- 

 tant, therefore, that in exact experiments precautions should be taken 

 to prevent the drying of living tissues. For this purpose the moist 

 chamber is used. It consists of a glass case to cover the myograph or 

 other apparatus employed for holding the tissue, and it is kept moist by 

 the presence of a piece of tow or cotton-wool soaked in water. 



The excitability of nerve is altered by changes of temperature. 

 Local cooling raises the excitability for most stimuli, but in order to 

 demonstrate this special precautions, as Gotch has shown, are neces- 

 sary. For, when a moist electrolyte is cooled its resistance is increased 

 and therefore with an electrical stimulus the physiological increase in 

 excitability may be completely obscured by the physical increase in 

 resistance. In order to eliminate this source of error it is only 

 necessary to stimulate the nerve by a galvanic current passing through 

 a large external resistance, 100,000 ohms; in this way any change in 

 the resistance of the nerve will be, when compared with the total 

 resistance of the circuit, so small as to be negligible. 



A freshly prepared muscle and nerve preparation is placed in the 

 moist chamber and under one portion of the sciatic nerve is placed 

 a small glass tube through which a current of warm or cold water 

 can be circulated. The make of a descending current is used as 

 the stimulus so that the kathode is upon that portion of the nerve 



