STIMULATION OF THE CRESCENT. 247 



recorder. Attach the thread by slipping it between the end of 

 the aluminium and the recording arm; friction will hold it firmly. 

 9 Adjust the counterpoising spring so that the tension may 

 be such as to give the maximal excursion to the lever ; see 

 that the latter is horizontal. Owing to the oblique position 

 of the spring, the tension remains almost constant in all 

 positions which the lever will assume. The heart may be 

 stretched to a considerable extent. Drum at slowest speed, 

 3 or 4 contractions to a cm. 



Take a record of the cardiac movements. The contractions 

 will be marked by down strokes, and the tracing may show 

 sinus, auricle, and ventricular contractions. The first is usually 

 absent, and the distinctness of the auricular curve is much 

 affected by the pericardial attachments, which vary a good 

 deal in different frogs. It is necessary to look for and divide 

 any restraining tissue if the movements appear hampered. 



Stimulation of the crescent. Find at the sinus-auricular junc- 

 tion, the curved tendinous line of demarcation which has its 

 convexity turned auriclewards. This is known as the crescent. 



Use the light pair of electrodes made of thin copper wires, 

 mounted on a cork transfixed by two pins by means of 

 which they are to be fixed to the frog plate. The ends 

 of the wires are to be placed so as to embrace the crescent, 

 apply interrupter shocks and gradually increase their strength. 

 Fairly strong shocks will be required. The heart will at 

 first beat faster for a few beats and then become slowed, 

 and if the stimulus be of sufficient strength, it will be 

 arrested in diastole, i.e., the lever will rise to and remain 

 at zero. If the stimulation be continued, the heart often 

 begins to beat again (vagus escape). 



The first few quicker beats are due to the vagus being 

 a vago-sympathetic (Gaskell), and the accelerator fibres are 



