LUDWIG AND COATS' APPARATUS. 307 



become full of fresh serum. The stopcock c is then turned so 

 as to cut off the tube c c' o!' from all communication with K ; . 

 and tracings are then taken, an abscissa or zero-line being drawn 

 under each. The heart is next supplied with poisoned serum 

 from k', and the tracings which it gives are compared witli the 

 normal ones. By slightly turning the stopcock c, a greater or 

 less resistance may be opposed to the circulation of fluid, and 

 tlie effects thus imitated which contraction or relaxation of the 

 vessels would produce in the living animal. 



Another apparatus has been invented by Ludwig, and used 

 by Coats in his research on the vagus, in which there is no 

 circulation, the serum being simply forced out of the ventricle 

 at each systole, aud falling back at each diastole. It gives, 

 however, very good tracings of the number and form of the 

 heart-beats, and is extremely well adapted for observations on 

 the effects of drugs on the vagus. It consists of a manometer, 

 E. and a reservoir. A, with which the frog's heart is connected by 

 two cannul?e, D and d'. The frog's heart is prepared by destroy- 

 ing the brain and spinal cord, removing the steriuim and fore- 

 legs, but leaving a large flap of skin, s, to cover the heart with, 

 and then introducing a cannula into the vena cava and aorta, as 

 in the former experiment. Instead of then cutting out the 

 heart, the liver and lungs are removed, and the stomach is cut 

 through the middle ; and a glass tube, sealed at both ends, and 

 as thick as the oesophagus will admit, is pushed through it till 

 one end projects at the mouth and the other from the cut end 

 of the stomach. The vagus is thus clearly displayed ; and, in 

 prder to isolate it more perfectly, all other nerves should be cut 

 away, as well as a part of the pharynx, so that no soft parts 

 may touch it from its exit from the bone to the place where it 

 crosses the aorta. From this point to the heart, it should be 

 left untouched ; and the jugular vein should not be tied, so as 

 to leave it undisturbed. The glass tube J is then fixed firmly 

 in a holder L, and the cannulse, D and d', connected with the 

 reservoir A, and the manometer E. Instead of the reservoir A 

 shown in the figure, it is perhaps better to use two fountain- 

 bottles. The apparatus is used just like that shown in Fig 136 ; 

 and the heart should in this case also be filled so full that a 



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