172 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



cut across, and the cut end raised on a thread in the air, is without effect on 

 the blood-pressure and pulse-rate. The stimulation of the central stumps of 

 these nerves, <>n the contrary, is followed by changes both in the blood-pressure 

 and the pulse, showing that they carry impulses from the heart to the cardiac 

 (•(litres in the central nervous system, or perhaps, according to the views of 

 some recent investigators, to peripheral ganglia, thus modifying the action of 

 the heart reflex ly. 



Sensory Nerves of the Heart. — The stimulation of intracardiac nerves 

 l>y the application of acids and other chemical agents to the surface of the 

 heart causes various reflex actions, such as movements of the limbs. The 

 afferent nerves in these reflexes are the vagi, for the reflex movements dis- 

 appear when the vagi are cut. On the strength of these experiments the 

 vagus has been believed to carry sensory impressions from the heart to the 

 brain. Direct stimulation of the human heart, in cases in which a defect in 

 the chest-wall has made the organ accessible, give evidence of a dim and very 

 limited recognition of cardiac events — for example, the compression of the 

 heart. Changes in the force, periodicity, and conduction of the contraction- 

 wave may be produced by direct electrical stimulation of the ventricle. The 

 centre of these reflexes probably lies in the bulb. 1 



Vagus. — The stimulation of the central end of the cut vagus nerve, 2 the 

 other vagus being intact, causes a slowing of the pulse-rate. The section of 

 the second vagus causes this retardation of the pulse to disappear, indicating 

 that the stimulation of the central end of the one affects the heart reflexly 

 through the agency of the other vagus. The blood-pressure is simultaneously 

 affected, being sometimes lowered and sometimes raised, the difference seeming 

 to depend largely on the varying composition of the vagus in different ani- 

 mals and in different individuals of the same species. The stimulation of the 

 pulmonary branches, by gently forcing air into the lungs, loud speaking, singing, 

 etc., is said to increase the frequency of the heart-beat. Yet the chemical 

 stimulation of the mucous membrane of the lungs is alleged to slow the pulse- 

 rate and lower the blood-pressure. Observers differ as to the results of stim- 

 ulation of the central end of the laryngeal branches of the vagus on the pulse- 

 rate and blood-pressure. 



Depressor Nerve. — The earlier stimulations of the nerves that pass 

 betweeD the central nervous system and the heart, with the exception of the 

 vagus, altered neither the blood-pressure nor the pulse-rate. Ludwig and Cyon 

 suspected that the negative results were owing to the fact that the stimulations 

 were confined to the end of the cut nerve in connection with the heart. Some 

 of the nerves, they thought, should carry impulses from the heart to the brain, 

 and such nerves could be found only by stimulation of the brain end of the 

 cut nerve. They began their research for these afferent nerves with the branch 

 which springs from the rabbit's vagus high in the neck and passes downward 

 to the ganglion stellatum. Their suspicion was at once confirmed. The stimu- 



1 Muskens: Archivfur die gesammte Physiologie, 1897, lxvi. p. 328. 

 -Jlmit: Journal of Physiology, 1895, xviii. p. 381. 



