A MANUAL OF PHYSIOLOGY 



the beat of a heart. For this purpose a tracing showing the speed 

 of the travelling surface in a given time is often taken simultaneously 

 with the record of the movement under investigation. For a slowly- 

 moving surface it is sufficient to mark intervals of one or two seconds, 

 and this is very readily done by connecting an electro-magnetic 

 marker (such as the electric signal of Deprez) with a circuit which is 



FIG. 215 SHORT-CIRCUITING 

 KEY. 



FIG. 216. TIME-MARKER. 



Arrangement for marking 2 second 

 intervals. D, seconds pendulum, 

 with platinum point E soldered on ; 

 A, mercury trough, into which E 

 dips at end of its swing ; B, Daniell 

 cell ; C, electro - magnets which 

 draw down writing-lever F when 

 the current is closed by E dipping 

 into A ; G, spring (or piece of india- 

 rubber), which raises F as soon as 

 current is broken. 



closed and broken by the seconds pendulum of an ordinary clock 

 (Fig. 216) or a metronome (Fig. 76, p. 179). For shorter intervals 

 a tuning.-fork is used, which makes and breaks a circuit including an 

 electromagnetic marker, or writes on the drum directly by means of 

 a writing-point attached to one of the prongs. 



In all the great functions of the body muscular movements 

 play an essential part. The circulation and the respiration, 

 the two functions most immediately essential to life, are kept 

 up by the contraction and relaxation of muscles. The move- 

 merits of the digestive canal, the regulation of the blood-supply 

 to its glands and to all parts of the body, and that immense class 

 of movements which we call voluntary, are all dependent upon 

 muscular action, which, again, is indebted for its initiation, 

 continuance, or control, to impulses passing along the nerves 

 from the nerve-centres. Hitherto we have not gone below 

 the surface fact, that muscular fibres have the power of con- 

 tracting, either automatically, or in response to suitable stimuli. 

 In this chapter and the two next we shall consider in detail 

 the general properties of muscle, nerve, and the other excitable 

 tissues. 



