30 ANIMAL MECHANISM. 



such motions as are produced by excitation, electric or 

 otherwise, applied to the portion of the nerve which remains 

 in communication with it. Ou the sides of the sciatic nerve 

 are an artery and a vein. Compression of the artery will 

 prevent the blood from reaching the muscle ; compression of 

 the vein will produce stagnation of the blood. The influences- 

 which different states of circulation produce upon the muscular 

 function may then be observed ; and, finally, by making an 

 incision in the skin of the foot, the muscle will be laid bare, 

 and cold, heat, or the various poisonous substances by which 

 its action is modified, may be brought to bear directly upon it. 



When the nerve of a frog thus prepared is excited by an 

 electric discharge, a very brief convulsive movement in the 

 muscle is produced; this motion is called Zuckung by the 

 German physiologists, and we propose to call it shock, in 

 order to distinguish it from true contraction. It is so rapid 

 that its phases cannot be distinguished by the eye, so that, to 

 appreciate its characteristics aright, recourse must be had to 

 special instruments. Registering apparatus only can supply 

 this need, for they faithfully render all the phases of motion 

 communicated to them. The general disposition of these forms 

 of apparatus, which for a long time were used almost exclusively 

 in the service of meteorology, is generally known. The 

 indications of the barometer, of the thermometer, of the force 

 or the direction of the wind, of the quantity of rainfall, &c. , 

 register themselves under the form of a curve which, accord- 

 ing as it is elevated or depressed, expresses the increase or 

 diminution of intensity of the phenomenon to be registered. 

 The time during which these variations are accomplished may 

 be estimated by the length occupied by the curve upon the 

 paper, which travels in front of the marking pen with an 

 ascertained and perfectly regular speed. 



The use of instruments of the same kind has been introduced 

 into physiology by Volkmann, Ludwig, and Ilelmholtz. We 

 have endeavoured to extend the employment of them to a great 

 number of phenomena, and we have constructed many instru- 

 ments whose d( si'.ription would be out of place here. The 

 apparatus which registers muscular motions bears the name 

 of myograph , it shows the disturbance of the muscle by 



