ADVANCED EXPERIMENTAL PHYSIOLOGY 91 



brass bridge of the key, which has many thousands of times less 

 resistance than the tissue 1 between the electrodes, affords a perfect 

 closure of the secondary circuit and prevents static electrification 

 of the electrodes. 



Errors from unipolar action are liable to take place, especially 

 in the study of the electromotive phenomena of muscle and nerve 

 by the electrometer and galvanometer. 



Galvani's Experiment with Metals. A piece of zinc is connected 

 with a piece of copper wire and the free end of one element is placed 

 under the sciatic nerve of a muscle-nerve preparation. If the 

 circuit be completed the nerve will be stimulated and the muscle 

 contract at make ; on breaking the circuit there will be a con- 

 traction at break. 



Mechanical Stimulation of the nerve can be shown by pinching 

 the nerve with a pair of forceps ; the muscle contracts, showing 

 that a nervous impulse was produced. Such a method of stimu- 

 lation injures the nerve, but by means of simple arrangements a 

 nerve can be stimulated mechanically without damage. A light 

 hammer worked by an electro -magnet may be used to tap the 

 nerve (see p. 128), or small drops of mercury from a funnel may 

 be allowed to fall upon the nerve. Such methods are useful in 

 experiments (see p. 128) in which an electrical stimulus might 

 introduce a source of fallacy, but for ordinary experiments they 

 are undesirable, since there is a difficulty in maintaining a constant 

 strength of stimulus, and there is a danger of damage to the nerve. 



Thermal Stimulation is next shown by the application of a hot 

 wire to the nerve. The muscle contracts. The damaged portion of 

 the nerve is cut away, and to the end of the living nerve is applied 

 a crystal of common salt ; the muscle soon shows irregular twitches 

 due to the chemical stimulation of its nerve (see p. 127). The 

 last form of stimulus is obviously limited to special experiments, 

 for the stimulus is not easily graduated and damages the nerve. 



CHAPTER II 



THE CONDITIONS WHICH AFFECT SINGLE MUSCULAR 

 CONTRACTIONS 



(a) Different Muscles, (b) Veratrine. The curve produced by 

 the contraction of a muscle may be altered not only by such influ- 

 ences as temperature, load, fatigue, and drugs, but also by the 

 differences in structure of various muscles. The muscular fibres 

 of the frog are found to present two varieties, clear and granular, 

 which differ both in structure and in physiological properties. 

 The gastrocnemius may be taken as an example of a muscle whose 



1 The resistance of a piece of a frog's sciatic nerve 1 cm. long is about 

 100,000 ohms. 



