VAGUS NERVE. 707 



with perfect ease. In the course of a few days the animal (carnivora) becomes 

 quieter, breathes less laboriously and the passive, flabby movement of the vocal 

 bands disappears. If, however, in the further course of events, even after a 

 considerable time, the animal is actively stimulated, there occurs in the presence 

 of the marked need for air an attack of extreme dyspnea, which subsides only 

 when the animal (dog) gradually becomes quieter. In consequence of the paralysis 

 of the larynx, foreign bodies may gain entrance into the trachea, particularly as 

 the paralysis of the uppermost portion of the esophagus renders swallowing diffi- 

 cult. In this way bronchopneumonia may develop. 



The depressor nerve, which in rabbits arises from the trunk of the 

 superior laryngeal and occasionally, with a second root from the trunk 

 of the vagus itself, passes with the sympathetic downward in the neck, 

 descends into the stellate ganglion and enters thence into the cardiac 

 plexus. It is a centripetal nerve, irritation of which, and also of its 

 central stump, diminishes the energy of the vasomotor center, so that 

 the blood-pressure falls. At the same time this irritation is conveyed 

 to the cardiac inhibitory center, so that the number of pulsations of the 

 heart diminishes. 



The depressor nerve is present also in the cat (Fig. 246, 77), the hedgehog, 

 the rat, and the mouse. In the horse and in man fibers analogous to the de- 

 pressor nerve pass back again into the trunk of the vagus. Also in the rabbit 

 fibers having a depressing effect may pass in the trunk of the vagus itself. The 

 depressor fibers of the rabbit enter the oblongata through the upper root-filaments 

 of the vagus. The inhibitory reflex for the heart is effective only upon the same 

 side. 



The branches of the vagus for the cardiac plexus (g, /), as well as 

 the latter itself, have already been described. They contain the in- 

 hibitory fibers for the movement of the heart (they are derived from 

 the most anterior root-filaments of the inner branch of the accessory 

 nerve), also sensory fibers for the heart (in the frog and in part in 

 mammals). Finally, the heart receives also through the vagus a 

 portion of its accelerator fibers ; feeble irritation of the vagus causes at 

 times acceleration of the heart-beat. In cases of poisoning with atropin 

 and nicotin, which paralyzes the inhibitory fibers, irritation of the vagus 

 causes acceleration of the heart -beat. The following experiment tends 

 to support the existence of vasomotor fibers in the cardiac branches : 

 Persistent irritation of the peripheral stump of the vagus causes ex- 

 travasation of blood into the endocardium (long-continued poisoning 

 with digitalin or strychnin has a similar effect), in consequence of spas- 

 modic contraction of the endocardial vessels, with secondary paralytic 

 relaxation and rupture. 



The pulmonary branches of the vagus are grouped together in the 

 anterior and posterior pu monary plexuses. The former supplies 

 sensory and motor branches to the trachea and passes then on the 

 anterior surface of the bronchial ramifications into the lungs (L). The 

 posterior plexus, formed of from three to five large branches derived 

 from the trunk of the vagus at the side of the bifurcation, anastomoses 

 with branches from the inferior cervical ganglion of the sympathetic and 

 with fibers of the cardiac plexus, and, after fibers from each side have 

 interchanged by decussation, passes with the branches of the bronchial 

 tree into the lungs. The pulmonary branches are supplied with gan- 

 glion-cells, as are also the larynx, the trachea, and the bronchi. From 

 the pulmonary plexus filaments pass to the pericardium and the superior 

 vena cava. 



