240 HANDBOOK OF PHYSIOLOGY. 



trunk, it may be understood that the effect of the stimulation of the 

 vagus in the frog is not in all cases purely inhibitory, but may be aug- 

 meutor, according to the position where the stimulus is applied, the 

 intensity of the stimulus, and the condition of the heart; if it is beating 

 strongly a slight vagus stimulation will produce immediate inhibition. 



The fibres of the vagus which pass to the heart arise in the medulla 

 oblongata, in the floor of the fourth ventricle, and in a nucleus of gray 

 matter, the exact position of which will be indicated in a future chap- 

 ter. It was formerly thought that the inhibitory fibres in the vagus 

 trunk were derived from the spinal accessory nerve, but this view has 

 been largely abandoned. The spinal accessory fibres probably supply 

 certain muscles of the larynx. It has been found that stimulation of 

 this nucleus, which is called the cardio-inhibitory centre, produces 

 inhibition of the heart-beat. 



Thus there is no doubt that the vagi nerves are simply the media of 

 an inhibitory or restraining influence over the action of the heart,, which 

 is conveyed through them from the centre in the medulla oblongata 

 which is always in operation. The restraining influence of the centre 

 in the medulla may be reflexly increased by stimulation of almost any 

 afferent nerve,, particularly of the abdominal sympathetic, so as to pro- 

 duce slowing or stoppage of the heart, through impulses from it passing 

 .down the vagi. As an example of this reflex stimulation,, the well- 

 jknown effect on the heart of a violent blow on the epigastrium may be 

 (referred to. The stoppage of the heart's action in this case is due to 

 the conveyance of the stimulus by fibres of the sympathetic (afferent) 

 to the medulla oblongata, and its subsequent reflection through the vagi 

 (efferent) to the muscular substance of the heart. It is possible that the 

 power of the medullary inhibitory centre may in a similar manner be 

 reflexly lessened so MS to produce accelerated action of the heart. 



The course of the augmentor fibres in the spinal cord is not known, 

 but it is thought that in all probability they are connected with an aug- 

 mentor centre in the medulla. The circulation of venous blood appears 

 to stimulate the inhibitory centre, and of highly oxygenated the aug- 

 mentor centre. 



In addition to direct and reflex stimulation it is almost certain that 

 impulses passing down from the cerebrum may have a similar effect. 



Other Influences Affecting the Heart-Beat. Alteration of tempera- 

 ture. The effect of cold is to slow the heart-beats, and if the heart, be 

 cooled down to 3 C. (38 F.) it will stop beating. The heart may be 

 frozen, and when thawed will continue its spontaneous beats. The 

 effect of heat is to quicken and shorten the heart-beats, but at a moder- 

 ate temperature, 20 0. (68F.), the contractions are increased in force. 



