668 STIMULATION OF THE TRUNK OF THE VAGUS. 



with during severe labour, this sufficiently explains the enfeeblement of the action 

 of the heart which occurs during protracted labour ; it is due to stimulation of the 

 central end of the vagus by the dyspnoaic blood (B. S. Schultze). (4) At the 

 moment tfie respiratory centre is excited, and an inspiration occurs, there is a 

 variation in the inhibitory activity-of the cardiac centre (Bonders, Pjiiiger, Frederico 



74, a. 4). (5) The centre is excited by increased blood-pressure within the 



cerebral arteries. 



II. The centre may be excited reflexly by (1) Stimulation of sensory nerves 

 ( Lovtn). (2) Stimulation of the central end of one vagus, provided the other vagus 

 is intact. (3) Stimulation of the sensory nerves of the intestines, by tapping upon 

 the belly (Goltz's tapping experiment), whereby the action of the heart is arrested. 

 Stimulation of the splanchnic directly (Asp anil Ludivig), or of the abdominal or 

 cervical sympathetic, produces the same result. Very strong stimulation of 

 sensory nerves, however, arrests the above-named reflex effects upon the vagus 

 ( 361, 3). 



Tapping Experiment. Goltz's experiment succeeds at once, by tapping the intestines of a 

 frog directly, say, with the handle of a scalpel, especially if the intestine has been exposed to 

 the air for a short time, so as to become inflamed (TarcJianojf). Stimulation of the stomach of 

 the dog causes slowing of the heart-beat (Sig. Mayer and Pribram). [M 'William finds that the 

 action of the heart of the eel may be arrested reflexly with very great facility. The reflex 

 inhibition is obtained by slight stimulation of the gills (through the branchial nerves), the 

 skin of the head and tail, and parietal peritoneum, by severe injury of almost any part of the 

 animal, except the abdominal organs.] 



[Effect of Swallowing Fluids. Kronecker has shown that the act of swallowing interferes 

 with or abolishes temporarily the ca'rdio-inhibitory action of the vagus, so that the pulse-rate is 

 greatly accelerated. Merely sipping a wine-glassful of water may raise the rate 30 per cent. 

 Hence, sipping cold water acts as a powerful cardiac stimulant] 



According to Hering, the excitability of the cardio-inhibitory centre is diminished by 

 vigorous artificial ventilation of the lungs with atmospheric air. At the same time, there is a 

 considerable fall of the blood-pressure ( 352, 8, 4). In man, a vigorous expiration, owing to 

 the increased intra-pulmonary pressure, causes an acceleration of the heart-beat, which 

 Sommerbrodt ascribes to a diminution of the activity of the vagi. At the same time the 

 activity of the vaso-motor centre is diminished ( 60, 2). 



Stimulation of the trunk of the vagus from the centre downwards, along its 

 whole course, and also of certain of its cardiac branches [inferior cardiac], causes 



! the heart either to beat more 



wwwwf r^s s e : r Tr^t ac dtu n s 



ear ea ' upon the strength of the stimulus 



j i 1 1 1 1 1 1 1 1 j i in m^u ^mn employed; feeble stimuli slow 



the action of the heart, while 



j stimulation j strong stimuli arrest it in dias- 



Fig. 468. tole. The frog's heart may be 



Beating of a frog's heart taken by means of a lever rest- arrested by stimulating the fibres 



ing on the heart. The lowest curve shows when the of the vagus upon the sinus ve- 



KC(S^ andtheC0,,8eqUentarrest0fthen <^ [<* by stimulating the 



vagus in its course as in fig. 

 468J. If strong stimuli be applied, either to the centre or to the course of the nerve, 

 for a long time, the part stimulated becomes fatigued and the heart beats more 

 rapidly in spite of the continued stimulation. If a part of the nerve lying nearer 

 the heart be stimulated, inhibition of the heart's action is brought about, as the 

 stimulus acts upon a fresh portion of nerve. 



fibres 6 ll0WiDg P intS haVe &h bCen ascertained regarding the stimulation of the inhibitory 



1. The experiments of Lowit on the frog's heart, confirmed by Heidenhain, showed that 



electrical and chemical stimulation of the vagus produce different results, as regards the extent 



of the ventricular systole, as well as the number of heart-beats ; the contractions either become 



