Physiology and biochemistry in modern medicine . erebrospinal axis. It is by the operation on the vagus center ofsuch impulses that changes in heart rate occur during emotional ex-citement, fright, etc. The increased heart rate in muscular exercise isprobably dependent upon a number of causes, such as the irradiation ofthe motor impulses on to the cardiac centers (see page 412), the rise intemperature and changes in the hydrogen-ion concentration of the blood,etc. Mechanism of Action of Vagus on the Heart.—Physiologists have nat-urally been curious as to the exact manner in which the vagus ner
Physiology and biochemistry in modern medicine . erebrospinal axis. It is by the operation on the vagus center ofsuch impulses that changes in heart rate occur during emotional ex-citement, fright, etc. The increased heart rate in muscular exercise isprobably dependent upon a number of causes, such as the irradiation ofthe motor impulses on to the cardiac centers (see page 412), the rise intemperature and changes in the hydrogen-ion concentration of the blood,etc. Mechanism of Action of Vagus on the Heart.—Physiologists have nat-urally been curious as to the exact manner in which the vagus nervebrings about inhibition of heart action. Similar inhibition as a resultof stimulation of efferent nerves exists in the case of the dilator fibersto the blood vessels (page 234) and the sympathetic nerve to the intes-tine (page 467). Inhibition of voluntary muscles can be produced onlythrough the central nervous svstem by stimulation of afferent nerves Left Ant. Caval vein .Riqht Ant Caval vein Preganglionic rreurOn • Postganglionic neuron. Inf Vena cava Hook fromHeart lever Bidders Ganglionin auriculo-ventricular junction stimulating electrodes in $mo-auricular junction [Crescent] Sympathetic fibres-- dotted lines Fig. 68. — Diagram to show the innervation cf the heart in the frog or turtle. The electrodesare represented as applied to the white crescentic line where they will stimulate some postganglionic libers. ( From Jackson.) THE CONTROL OF THE CIRCULATION 225 (page 814). It is not the nerve fibers themselves that are responsible for the inhibitory effect, for it has been found that if the peripheralend of a cut vagus nerve is connected with the central end of one ofthe anterior roots of the cervical portion of the spinal cord, the axonsof the latter when they grow down into the vagus trunk during theregeneration which follows, stimulation of the regenerated fibers willstill produce inhibition of the heart. The nature of the fibers can nottherefore be the factor upo
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