CIRCULATION. 159 



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. 



A tonic contraction 1 of the heart muscle is sometimes produced by strong, 

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

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

 poisons, such as digitalin and veratrin. 



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 < if view it is the 

 middle cervical sympathetic ganglion. The true inferior cervical sympathetic 

 ganglion is fused with the first one or two thoracic ganglia to form the gan- 

 glion stellatum, situated opposite the first intercostal space. At the " inferior 

 cervical" ganglion the vagus and the sympathetic part company, the vagus 

 passing caudalward behind the root of the lung and the sympathetic passing 

 to the stellate ganglion, dividing on its way into two portions (the annulus of 

 Yieussens), which embrace the subclavian artery. In many cases the lower 

 loop of the annulus of Vieussens joins the trunk of the vagus caudal to the 

 ganglion. 



The cardiac nerves spring from the vagus and the sympathetic nerve in 

 the region of the inferior cervical ganglion. They may lie divided into an 

 inner and an outer group. 



The inner group is composed of one medium, one thick, and two or 

 three slender nerves. The nerve of medium thickness springs fr the gan- 



1 Hunt, Bookman, and Tierney: CeniraJblait fur Physiologic, 1897, xi. p. 274. 



