114 THE CARDIAC NERVES. 



vation has been made in executed criminals. In the opened heart the 

 papillary muscles fail to contract synchronously with the auricular wall 

 after from two to three minutes. Engelmann made the interesting 

 observation that the muscles of the auricle may lose their power of 

 contracting, in response to irritation of the vagus or as a result of immer- 

 sion and swelling in water, without losing the power of conducting 

 stimuli. An analogous phenomenon has been observed- with respect 

 to the nerves. 



After the heart has ceased beating altogether, it can be temporarily 

 roused by direct stimulation, especially by heat; and again the auricles 

 and auricular appendages are the last to react. As a rule, when the heart 

 has been temporarily stimulated to greater activity it ceases to beat the 

 earlier; before the orderly succession of beats ceases altogether tremu- 

 lous, "undulating" movement of the muscle-bundles usually takes 

 place. In mammals, when the irritability of the heart has ceased, it 

 can be temporarily restored by injecting arterial blood into the coronary 

 vessels. In the frog the heart, which at first becomes rigid, may be 

 revived by filling its cavities with fresh blood. As the heart uses up 

 oxygen and eliminates carbon dioxid, it is quite conceivable that it 

 should beat longer in oxygen than in nitrogen, hydrogen, carbon dioxid, 

 hydrogen sulphid or in a vacuum, even when, to avoid desiccation, 

 aqueous vapor is generated in the vacuum. When the heart, after it 

 has ceased to beat, is returned to a medium containing oxygen, it 

 begins to beat again. 



THE CARDIAC NERVES. 



The cardiac plexus is formed by : i . The cardiac branches of the trunk of the 

 vagus nerve ; these include cardiac branches from the external branch of the supe- 

 rior laryngeal nerve, the inferior laryngeal nerve, and sometimes the pulmonary 

 branches of the vagus, in larger number on the right than on the left side. 2. The 

 superior, middle, inferior, and lowest cardiac branches from the three cervical 

 ganglia and the first thoracic ganglion of the sympathetic nerve, which frequently 

 vary in number and in size (sometimes one of the branches accompanies the 

 descending branch of the hypoglossus for a part of its course) . The branches of 

 the plexus are the deep and the superficial nerves; the latter usually contain a 

 ganglion at the bifurcation of the pulmonary artery beneath the arch of the aorta. 

 The following structures are regarded as belonging to the cardiac plexus: 



(a) The right and left coronary plexuses, which convey the vasomotor nerves 

 of the coronary vessels through the vagus portion and the dilators through the 

 sympathetic; and in addition contain sensory fibers derived from the vagus and 

 passing principally to the pericardium. In patients suffering from disease of the 

 heart the presence of sensory nerves is indicated by the occurrence of constant 

 or paroxysmal pain. In the frog, reflex phenomena may be induced from the 

 ventricle in the various portions of the heart, and they probably have their reflex 

 center in the medulla oblongata. 



(6) The nerves embedded in the heart-muscle and in the furrows, which are 

 richly supplied with ganglia and which have been designated the automatic 

 motor centers of the heart. The heart contains a circle of nerves richly supplied 

 with ganglia at the edge of the interauricular septum and another at the junction 

 of the auricles and the ventricles. Wherever the two meet they exchange fibers. 

 The ganglia are for the most part found near the pericardium. In mammals the 

 two larger ganglia are situated close to the orifice of the superior vena cava; in 

 birds the largest node of nerve-tissue, containing thousands of ganglia, occupies 

 the posterior point of decussation of the longitudinal and transverse sulci. These 

 nodes of nerve-tissue send smaller branches into the muscular walls of the auricles 

 and ventricles, and these branches in turn are the seat of smaller ganglia. 



In the frog a large collection of ganglia, Remak's ganglion, is situated, together 

 with the vagus fibers, within the wall of the sinus of the vena cava (the dilated 

 orifice of the venag cavas in the right auricle whose independent movement pre- 



