72 THE HEART 



fibres. These in the rabbit form a separate nerve, the 

 depressor nerve, in the neck, but in most animals are in- 

 corporated throughout mth the vagus. On stimulating 

 the central end of the depressor nerve in the rabbit, there 

 occur slomng of the heart and fall of blood-pressure, the 

 former due to impulses travelhng down the vagus, the 

 latter to dilatation of the peripheral blood-vessels — chiefly 

 those of the abdomen. 



We saw that in the heart-lung preparation, rise in the 

 arterial pressure, though it caused dilatation of the heart 

 with unaltered output, did not affect the frequency. 

 Wlien the arterial pressure is increased in the intact animal, 

 the heart is slowed (Marey's Law). The rise in pressure is 

 a stimulus to the depressor nerve-endings. Here then is 

 a protective mechanism whereby the heart is eased of a 

 load which is too great for it. 



But the afferent fibres do not all stimulate the vagus 

 centre. We saw that in the isolated heart, the output 

 increased with the venous inflow, but the frequency of the 

 beat was unchanged. There is evidence to show that 

 abnormal distension of the right auricle stimulates efferent 

 nerve-endings to produce reflex quickening of the heart. 



Reflexes originating in the Lungs. — -The beat is quickened 

 during inspiration, and slowed during expiration. This 

 phenomenon, which is known as sinus arrythmia, is abolished 

 when the vagi are cut. In children it occurs with normal 

 breathing ; in adults usually only during excessive respira- 

 tory movement. 



The Influence of the Higher Centres upon the Medulla 



Certain mental states, such as strong emotions, affect the 

 cardiac centre directly. The quickening of the beat 

 which occurs at the beginning of exercise is also produced 

 by the direct action upon the medulla of impulses originat- 

 ing in the cerebral centres and called into play by the 

 psychological process of attention. 



