THE CIRCULATION OF THE BLOOD. 241 



At or below 40 C. (104 F.) the contrations are so rapid as to pass into 

 heart-rigor; this may be stopped by cooling. 



Poisons and other chemical substances. A large number of 

 chemical substances have a distinct effect upon the cardiac contractions. 

 Of these the most important are atropin and muscarin. 



Atropin produces considerable augmentation of the heart-beats, and 

 when acting upon the heart prevents the results of vagus stimulation. 



Muscarin (obtained from various species of poisonous fungi) pro- 

 duces marked slowing of the heart-beats, and, in larger doses, stoppage 

 of the hearfc. It produces a similar effect to that of prolonged vagus 

 stimulation, and as in that case the effect can be removed by the action 

 of atropin. 



Diyitalin (the active principle of digitalis purpurea), slows the heart 

 and appears to act by stimulating the vagi. Later on the muscle is also 

 more excitable. Veratrin and aconitin have a similar effect. Nicotine 

 prevents the effect of vagus stimulation. 



Methods of investigating the Heart-beat. 



(1) The simplest form of an apparatus to be used for recording the contrac- 

 tions of the frog's heart consists of a small closed cylindrical box fixed to a 

 stand. At the bottom of the box are two tubes by means of which water at 

 various temperatures may be made to circulate through it, one tube being the 

 inlet and the other the outlet, and they are connected with india-rubber, suit- 

 able for the purpose of conducting water to and from the apparatus. The lever 

 is made of a piece of glass rod which is softened and drawn out in the flame 

 of the blow-pipe a very fine thread, leaving a small piece unaltered to act as 

 a counterpoise. The lever is then passed through a piece of cork and through 

 this cork a fine needle is inserted at right angles to the lever. The needle is 

 made to rest on a support attached above one side of the box in such a way 

 that the lever has free movement up and down. Another small piece of cork 

 is passed along the lever arm and is adjusted and cut so that its point directed 

 downward can rest upon the frog's heart, which is removed from the body and 

 placed upon the top of the box in serum or defibrinated blood. In this way 

 the contractions of the auricles and ventricle are communicated to the lever, 

 and this may be made to write upon a recording cylinder. 



(2) The variations of endocardial pressure, which correspond, of course, 

 with the various phases of the cardiac cycle, may be recorded by a modifica- 

 tion of the ordinary mercurial manometer. The apparatus is best used witli a 

 large frog (Rana esculenta) , and the heart is exposed in the usual manner, the 

 pericardium opened. A cut is made into the bulb, and by this means a double 

 or perfusion canula (fig. 196) is passed into the ventricle, a ligature is passed 

 round the heart, and the canula is tied in tightly. The vessels are then di- 

 vided beyond the ligature, and the canula, with the heart attached, is removed. 

 To one stem of the canula a tube is attached, communicating with a reservoir 

 of a solution of dried blood in . 6 saline solution, and filtered, which is capa- 

 ble of being raised or lowered in temperature by being surrounded by a metal 

 box which contains hot, cold, or iced water. Attached to the other end is a 



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