26 EXPERIMENTAL PHYSIOLOGY 



tion then varies inversely as the distance of the point of attachment 

 from the axis. Fig. 31 represents such a SIMPLE LEVER l, which 

 is represented as arranged for recording the movement of a muscle m, 

 whose upper end is held firmly in the MUSCLE FORCEPS p. All 

 recording levers should be made as light as possible, consistent with 

 sufficient rigidity to prevent distortion of the record by vibrations set 

 up in the lever itself. The question of lightness is of the greatest 

 importance when rapid movements are to be recorded. 



Another form of lever which is also very commonly employed is 

 the CRANK LEVER. This consists of a lever with two arms fixed at 

 right angles to each other. It is represented in fig. 37, p. 31, as 

 being used for recording a simple twitch of a muscle. One of the two 

 arms is long, and when used is fixed horizontally. This is the writing 

 lever. The other is fixed vertically, is much shorter, and is the lever 

 to which the muscle is attached. The muscle in this instance lies 

 horizontally, so that with a crank lever the movement of the writing 

 point is at right angles to the direction of the movement recorded. 



When we wish to excite a muscle electrically it is necessary to have a 

 pair of electrodes by which the shock may be carried to the muscle or nerve 

 to be excited. There are very many forms of these which can be employed. 



A simple form, readily made, is 

 shown in fig. 32. Two very thin 

 and flexible copper wires, a and b, 

 covered with silk are taken and 

 twisted together at d and e. A 

 small cork c is taken transfixed bv 



a pin p and two shallow cuts made 

 in it. The wires are then forced 

 into the cuts, as seen in the figure. 

 This holds the wires firmly. Near 

 the ends of the wires a drop of 

 Fig. 32.-Si.iple Form of Wire'' me ! te ? filing-wax w is fixed, so 



Electrodes as *° hold the wires parallel to one 



another and about 1 mm. apart. 

 The wires are then cut off about 4 mm. beyond the wax, and these projecting 

 pieces bared by scraping off the silk insulation. In many cases it is a 

 further advantage to imbed the uncovered points in wax, and only expose 

 the wires for about 1 mm., and on one surface only. This tends to prevent 

 escape of the current to surrounding parts. 



MINIMAL AND MAXIMAL EXCITATION 



If the strength of the excitation be varied, it is found that the 

 response of the muscle varies in amount. This should be studied in 

 the following way. 



Experiment 5. — Cover and blacken a drum. Dissect out a muscle and 

 attach it either to a simple-lever or to a crank-lever myograph. Fit up the 



