84 THE INVOL UNTAR Y NER VO US S YSTEM 



ventricular tissue and along the nerve fibres passing to the 

 auricles from the sinus, namely the auriculo-ventricular junction 

 or inter-auricular nerves. The main mass of the auricular or 

 ventricular muscles as well as the muscles of the bulbus arteriosus 

 are free from nerve cells. 



We find further that when, as in the tortoise, the fibres of the 

 vagus nerves in their passage from the sinus to the ventricle do 

 not travel in the septum between the two auricles, there are no 

 nerve cell groups in the septum, the nerve cells being only 

 found in connexion with the nerve fibres. I conclude that all 

 the nerve cells in the heart have come in with the cardiac branches 

 of the vagus nerve. 



As already stated, Dogiel has found in the mammalian heart 

 that the medullated fibres, which enter the heart, do not terminate 

 around the nerve cells of one ganglionic group, but individual 

 fibres can be traced to a considerable distance after they have 

 made connexion with one group of nerve cells before they ter- 

 minate in arborisations around a final group of cells ; in their 

 course they give off collaterals to other groups of cells. There 

 is some evidence that these are true connector fibres, in that 

 axon reflexes can take place through them ; in the tortoise 

 Testudo grcBca I found that one of the coronary veins fre- 

 quently passed free from the rest of the heart between the 

 ventricle and sinus, so that the auricles could be cut away 

 from the ventricle and still leave the latter in connexion with the 

 sinus by this vein. As a rule a small nerve, to which I gave the 

 name ' coronary nerve ' accompanies this vein. The fibres of 

 this nerve are medullated and belong to the vagus (usually the 

 right vagus). When the ventricle is thus isolated, I found that, 

 with the electrodes placed on the ventricle itself or on this 

 coronary nerve close to the ventricle, I could affect the rhythm of 

 the sinus and the contractions of the auricles even when the 

 current was so weak as not to cause any contraction of the ven- 

 tricle. Such effect was always of an inhibitory nature, shown by 

 slowing of rate and diminution in the force of the auricular con- 

 tractions. This effect is most probably due to an axon reflex of 

 the same nature as has already been described in the connector 

 fibres of the sympathetic. 



This same peculiarity of the tortoise heart enabled me to give 

 evidence that the nerve fibres, which pass from these vagus cells 



