1060 PHYSIOLOGY 



circulation of the blood is maintained by the contraction of all the 

 parts of the heart except the apex, which never resumes its activity. 

 If, however, the intraventricular pressure be raised by clamping the 

 aorta the apex begins to beat at its own rhythm, which is independent 

 of the rhythm of the rest of the heart. Moreover a strip can be cut 

 from the apex of the tortoise's ventricle (Fig. 425), free from ganglion- 

 cells, which on keeping in a moist chamber and moistening occasion- 

 ally with normal salt solution enters into rhythmic contractions. 



(b) In the frog it is possible to excise the inter-auricular septum 

 with its ganglia, and a considerable portion of the ganglia in the sinus 



venosus and at the base of the 

 ventricles, without interfering 

 ~ in any way with the cardiac 

 rhythm. This experiment is 

 still easier to carry out in the 

 tortoise's heart, where the 

 nerves and ganglia run in the 

 basal portion of the auricles. 

 This can be excised, leaving 

 the two auricular appendages 

 in connection with the sinus 

 venosus and with the ven- 

 tricles. 



(c) The heart in the develop- 

 ing chick can be seen beating 



FIG. 425. Tortoise's heart from dorsal surface, at a time when it is quite free 



(GASKELL.) from nerve-cells, which only 



S, sinus; J, sino -auricular junction; A, , . . , -, 



auricles; C, coronary vein; V, ventricle. (The extend into it at a later date, 



dotted line shows ho w a strip may be cut from (fl\ Remak's ganglia are 



the ventricle apex.) . v , , . 6 , 



situated at the point where the 



two vagus nerves enter the heart, and under the microscope can be seen 

 to be connected with the fibres of these nerves. We have now, from the 

 discovery of Landley and Dickinson, a means of judging of the action 

 of ganglion-cells in the drug nicotine, which first stimulates and then 

 paralyses nerve-cells themselves or the synapses between the cells and 

 the nerve fibres in connection with them. Direct application of nicotine 

 to the heart, after a primary period of slowing, leaves the heart-beat 

 practically unaltered, the normal sequence of beat in the various 

 cavities being unaffected. After the application of the drug, however, 

 stimulation of the trunk of the vagus is without effect, though slowing 

 or stoppage of the heart may still be produced by excitation of the 

 post ganglionic nerve fibres of the vagus which arise from the cells of 

 Remak's ganglia. These ganglia must therefore be regarded not as a 

 motor centre for the heart, but merely as a distributing-centre for the 



