450 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the extra stimulus falls after the contraction maximum or during diastole an 

 extra contraction results, otherwise not. The refractory period exists, there- 

 fore, independently of the cardiac ganglia. 1 



The compensatory pause can also, though not always, be secured with the 

 ganglion-free apex. 2 



The refractory period has been used to show how a continuous stimulus 

 might produce a rhythmic heart-beat. The continuous stimulus cannot affect 

 the heart during the refractory period from the beginning to near the maxi- 

 mum of systole. At the close of the refractory period the constant stimulus 

 becomes effective, causing an extra contraction with long latent period. This 

 latent period is, according to this theory, the interval between the first and the 

 second contraction. 3 



A tonic contraction of the heart muscle is sometimes produced by strong, 

 rapidly repeated induction shocks 4 and by various other means, such as filling 

 the ventricle with old blood, 5 by weak sodium hydrate solution, 6 and by certain 

 poisons, such as digitalin and veratrin. 7 



A. THE CARDIAC NERVES. 



The cardiac nerves are branches of the vagus and the sympathetic nerves. 



In the dog the vagus arises by about a dozen fine roots from the ventro- 

 lateral aspect of the medulla- and passes outward to the jugular foramen in 

 company with the spinal accessory nerve. In the jugular canal the vagus 

 bears a ganglion called the jugular ganglion. The spinal accessory nerve 

 joins the vagus here, the spinal portion almost immediately leaving the vagus 

 to be distributed to certain muscles in the neck, while the medullary portion 

 passes to the heart through the trunk ganglion and thereafter in the substance 

 of the vagus. Directly after emerging from the skull, the vagus presents a 

 second ganglion, fusiform in shape and in a fairly large dog about one centi- 

 meter in length. From the caudal end or middle of this " ganglion of the 

 trunk" is given off the superior laryngeal nerve, slightly behind which a 

 large nerve is seen passing from the sympathetic chain to the trunk of the 

 vagus. This nerve is in reality the main cord of the sympathetic chain, the 

 sympathetic nerve being bound up with the vagus from the " inferior " cervical 

 ganglion to the point just mentioned. Posterior to the trunk ganglion of the 

 vagus, the vago-sympathetic runs caudalward as a large nerve dorsal to the 

 common carotid artery as far as the first rib or near.it, where it enters the 

 so-called inferior cervical ganglion. This ganglion belongs to the sympathetic 

 system and not to the vagus ; from a morphological point of view it is the 

 middle cervical sympathetic ganglion. The true inferior cervical sympathetic 



1 Dastre, 1882, p. 447 ; Kaiser, 1895, p. 449 ; Engelmann, 1895, p. 326 ; compare Kronecker, 

 1875, p. 181. 



2 Kaiser, 1895, pp. 449, 457 ; Engelmann, 1895, p. 311 ; Dastre dissents, 1882, p. 464. 



3 Tigerstedt, 1893, p. 169. 4 Engelmann, 1882, p. 453. 



5 Aubert, 1881, p. 381 ; compare Rossbach, 1874, p. 97. 



6 Gaskell, 1880, p. 53. 7 Roy, 1879, p. 477. 



