978 PHYSIOLOGY 



pressure in the ventricle therefore rises to a marked extent (see Fig. 451). 

 The strain on the ventricular wall of this sudden contraction which is 

 necessary to empty the heart, during the period of systole, is often so great 

 that small haemorrhages are produced throughout the substance of the 

 muscle. The stimulation effect of adrenaline is shown, moreover, by the 

 considerable rise in the respiratory exchanges of the heart under the influence 

 of this substance, the oxygen intake being increased two or three times above 

 that which obtained before the administration of the adrenaline. The action_ 

 of adrenalin therefore is in general to enable the heart to cope with a bigger 

 strain ^either in the shape of arterial resistance or increased venous, inflow 

 than it could do without the stimulus of this substance. 



The wonderful adaptation of the heart to its functions is illustrated, 

 moreover, by the fact that adrenaline, which increases the metabolism of the 

 heart to such an extent, exercises at the same time a dilator effect on the 

 coronary vessels, so that apart from the high arterial pressure and the 

 metabolites produced by the contracting heart muscle, the vessels are 

 dilated by the action of the same hormone which evokes the need for an 

 increased flow of blood through the working muscle. 



There is thus a marked antagonism in the influence of the two common 

 hormones on the heart, both of them being produced during general muscular 

 activity. Carbonic acid in excess causes dilatation of the heart, diminished 

 functional activity, slowing of rhythm. Adrenaline causes increased func- 

 tional activity, diminution of cardiac volume, and increased rhythm. The 

 action of adrenaline is so pronounced that it is possible to administer 20 or 

 30 per cent, carbonic acid to a heart-lung preparation without altering 

 its output if adrenaline be administered at the same time. The heart is 

 slowed by the carbonic acid, but the beat is maintained and contraction is 

 effective in emptying the heart of its content. 



THE HEART REFLEXES 



The part of the nervous system chiefly concerned in the central co- 

 ordination of the various afferent impulses which act on the heart is the 

 medulla oblongata. It is in this situation that we find the nerve-cells giving 

 origin to the efferent fibres of the vagus nerves, and also the collection of 

 grey matter in which the afferent fibres of the vagus terminate. Direct 

 stimulation of the vagus centre may cause slowing and stoppage of the 

 heart. The tonic influence of the vagi can be abolished by destruction of this 

 centre. In this region we also find the vaso-motor centre, so that the 

 activity of one can affect that of the other. This cardiac centre may be 

 played upon by impulses arriving at it through various afferent nerves or from 

 the higher parts of the brain and giving rise to the changes of the pulse-rate 

 associated with the emotional conditions, or it may be directly affected by 

 the composition of the blood circulating through its capillaries. 



The nerve-cells which give off the accelerator or augmentor fibres are 

 situated in the intermedio-lateral tract of the spinal cord, near the point of 

 origin of these fibres. We might therefore speak of an augmentor centre in 





