472 APPENDIX 



one of the efferent wires also to one of these posts, and the other in the 

 post to which the movable block is hitched. Then when the block is at 

 the post containing two wires none of the incoming current is taken off 

 to the nerve or muscle; when the block is half-way down the german- 

 silver wire half the current passes out, and when at the opposite corner 

 post all of it. To increase or decrease the current gradually, merely 

 slide the block along the german-silver wire in the direction required. 

 (See Expt'44.) 



The electromagnetic signal is always placed in the primary or battery- 

 circuit and cannot be actuated by the induction-current. It should 

 write immediately beneath the myograph pen. 



The tuning-fork makes one-hundred double vibrations per second 

 and vibrates fifteen or twenty seconds at a time. It is to be held in the 

 hand and set in action by pinching together the bars and releasing them 

 suddenly. The writing pen is attached to the edge of the end of one bar. 



The kymograph or movement-recorder has several speeds. By means 

 of the screw on top of the drum-spindle the drum can be raised off the 

 friction-bearing and may then be spun by hand independently of the 

 clock-work. Keep the kymograph well wound up in order to obtain 

 approximate constancy of speed. The writing-pen should always be 

 at a tangent to the surface of the drum, which must go in the right direc- 

 tion in reference to the tinsel pen and never so as to tend to double it up. 

 In other words, have the kymograph always to the left of the remainder 

 of the apparatus. 



IV. THE MECHANICS OF THE CIRCULATION. 



Expt. 22 consists of a demonstration of the various parts of the 

 circulation-schema, what they correspond to in the animal, and how to 

 use the apparatus in the laboratory. 



FIG. 258 



Brachial sphygmograms. Made on the brachial artery with the simple laboratory thistle- 

 tube sphygmograph. To be read from left to right. Time-line is in seconds. 



The bulb, representing the ventricles, must always be compressed in 

 the palm of the hand at the rate at which the human heart works, using 

 a watch or the actual pulse as a guide to the correct rate. (A newer 

 form of the circulation schema on the market has no bulb, but an excen- 

 tric crank-mechanism in its place. The valves furthermore can be 

 more readily changed, making it preferable.) The valve-flaps should 

 be of thin rubber dam. The capillary resistance must be so arranged 



