126 APPLIED PHYSIOLOGY 



at birth,* but their tone increases up to twenty-five to 

 thirty-five years, and from this time lessens again, and 

 is very slight in old age. Atropine, which acts by 

 paralyzing the ends of the inhibitory fibres, has thus 

 very little effect on the heart at the extremes of life. 

 This may be one reason why children stand large doses 

 of belladonna so well. 



Afferent Impulses. 



The course of the afferent nerves from the heart to 

 the nervous system is not so well known as that of the 

 efferent. In man they appear to be both in the vagus 

 and in the rami communicantes of the upper dorsal 

 nerves.t The vagus fibres which constitute the de- 

 pressor nerve are those with which we are best ac- 

 quainted, though these probably belong, in man at least, 

 to the root of the aorta rather than to the heart itself. 

 The name ' depressor ' is applied to these fibres because 

 of the great lowering of general blood-pressure which 

 stimulation of them brings about. They appear to be 

 thrown into action when the heart is struggling against 

 an aortic pressure which is rather too great for it to 

 overcome, and their stimulation appears to be attended 

 by the production of pain, which may manifest itself in 

 some cases in the form of the disease known as angina 

 pectoris. Normally, however, the heart, like most of 

 the internal organs, appears to be but poorly endowed 



* Cushny, 'A Textbook of Pharmacology,' first edition, 1900, 

 p. 280. 



f See Ferrier's Harveian Oration on 'The Heart and Nervous 

 fiystem,' Brit. Med. Journ., 1902, ii. 1336. 



