344 PRACTICAL PHYSIOLOGY 



output of the heart. This is owing to the expulsion of blood from 

 the thorax. Thus each systole aspirates air into the lungs. The same 

 thing may be demonstrated by putting a U-shaped tube in the nostril 

 filled with tobacco smoke. The column of smoke moves in and out 

 with each systole and diastole of the heart. 



CHAPTER XXIV. 

 THE TORTOISE HEART. 



The Tortoise Heart. Pith the brain of the land-tortoise (Testudo 

 graeca), saw through the sides of the carapace, and remove the lower 

 half of it. The heart will now be exposed. Pin out the head of the 

 animal, and look for the vagus nerve which runs in company with the 

 carotid artery on either side of the trachea. Record the heart by 

 the suspension method, ligature the vagus, divide it, and excite 

 the peripheral end. The heart will be arrested in diastole, and may 

 remain in arrest for more than one minute. Excise the heart, cutting 

 widely round the sinus, and lay it on a clean glass plate. Note 



FIG. 223. Diagram of the tortoise FIG. 224. Tortoise heart with auricles slit 



heart as suspended for recording. N = so as to pi-oduce a block, 



coronary nerve. 



the sinus (Fig. 224), the two auricles, the single ventricle, and 

 the 'sinus extension' which connects the ventricle with the sinus. 

 The main groups of ganglion cells are found at the bifurcation of the 

 large vagus nerve trunks in the sinus (corresponding to Remak's 

 ganglion), in the sinus extension (V. Bezold's), and in the termination 

 of this in the auriculo-ventricular ring (Bidder's). The auricles and the 

 ventricle are ganglion-free, except in the neighbourhood of the sinus 

 extension and auriculo-ventricular ring. One of the coronary veins 

 runs on the surface of the sinus extension from ventricle to sinus 



