MUSCLE-NERVE PHYSIOLOGY. 527 



ds, and the legs on the thighs, and then suddenly straightening out the angles 

 thus formed. The share which this action has in producing the effect may be 

 iasily known by attempting to leap in the upright posture, with the legs quite 

 straight. 



Running is performed by a series of rapid low jumps with each leg alter- 

 nately ; so that, during each complete muscular act concerned, there is a moment 

 when both feet are off the ground. 



In all these cases, however, the description of the manner in \vhich any 

 ^iven effect is produced, can give but a very imperfect idea of the infinite 

 number of combined and harmoniously arranged muscular contractions which 

 are necessary for even the simplest acts of locomotion. 



Action of the Involuntary Muscles. The involuntary muscles 

 ire for the most part not attached to bones arranged to act as levers, but 

 3nter into the formation of such hollow parts as require a diminution of 

 iheir calibre by muscular action, under particular circumstances. Ex- 

 mples of this action are to be found in the intestines, urinary bladder, 

 heart and blood-vessels, gall-bladder, gland-ducts, etc. 



The difference in the manner of contraction of the striated and non- 

 itriated fibres has been already referred to (p. 529) ; and the peculiar 

 vermicular or peristaltic action of the latter fibres has also been described. 



ELECTRICAL CURRENTS IN NERVES. 



The electrical condition of nerves is so closely connected with the 

 phenomena of muscular contraction, that it will be convenient to con- 

 sider it in the present chapter. 



If a piece of nerve be removed from the body and subjected to exami- 

 nation in a way similar to that adopted in the case of muscle, which has 

 been described, electrical currents are found to exist which correspond 

 exactly to the natural muscle currents, and which are called natural 

 nerve currents or currents of rest, according as one or other theory of 

 their existence be adopted, as in the case with muscle. One point 

 (equator) on the surface being positive to all other points nearer to the 

 :ut ends, and the greatest deflection of the needle of the galvanometer 

 taking place when one electrode is applied to the equator and the other 

 to the centre of either cut end. As in the case of muscle, these nerve 

 currents undergo a negative variation when the nerve is stimulated, the 

 variation being momentary and in the opposite direction to the natural 

 currents; and are similarly known as the currents of action. The cur- 

 rents of action are propagated in both directions from the point of the 

 application of the stimulus, and are of momentary duration. 



Rheoscopic Frog. This negative variation may be demonstrated by means of 

 the following experiment. The new current produced by stimulating the nerve 

 of one nerve-muscle preparation may be used to stimulate the nerve of a second 

 nerve- muscle preparation. The foreleg of a frog with the nerve going to the 



