THE HEART 59 



maintain the tonic activity of the cardio-inhibitory centre. It 

 can certainly be shown experimentally that every contraction of 

 the heart sends a series of impulses up this nerve, but whether 

 these are conveyed to the cardio-inhibitory centre has not been 

 decided. 



We have now seen how complex are the influences exercised 

 by the nervous system in connection with the rhythm, force, 

 and tone of the heart muscle, the moderating influence in these 

 directions exercised by the vagus, and the stimulating functions 

 of the sympathetic. The factors determining which influence 

 at the moment is best suited to the existing conditions originate 

 in the periphery, and not in the central nervous system ; from 

 the periphery — viz., skin, muscles, viscera, bloodvessels, and 

 heart — impulses are transmitted to the brain which are obeyed 

 by the cardio-inhibitory or accelerator centres in the medulla. 



The Chemical Stimulus. — One further feature in connection 

 with the matter still remains for consideration, and that is the 

 influence of certain ' internal secretions,' on increasing or dimin- 

 ishing the function of the heart muscle. It is now known that 

 the adrenal capsules and pituitary body are capable of furnishing 

 extracts which, when introduced into the circulation, exercise 

 a powerful effect on the heart. An extract of the medulla of 

 the adrenal capsules increases the rhythm, force, and tone of 

 cardiac muscle ; an extract of pituitary body contains two 

 substances : one increases the tone, but does not affect the rate 

 of rhythm of the muscle, while the second substance diminishes 

 the force of the heart-beat. It may well be that these ductless 

 glands pour into the circulation their various secretions in the pro- 

 portions suited to the most efficient working of the heart, and by 

 their action on the nerve-centres maintain them in effective 

 working order.* The action of these internal secretions on the 

 movements of the bloodvessels will be referred to later. 



Pathological. 



Disease of the heart of the lower animals is uncommon. It might 

 have been thought that horses would be particularly exposed to this 

 class of trouble, bearing in mind the enormous strain placed on the 

 heart during labour, and the utter want of consideration shown by 

 the vast majority of those who ride and drive them. But it is not so. 

 The hearts of horses exposed to the greatest strain seldom show any 

 pathological change ; probably the most uncommon lesions found 

 on post-mortem examination are those affecting the heart. The 

 heart may dilate under strain, but such^dilatation when accompanied 

 by hypertrophy is compensated, and no indication of trouble exists 



* See ' The Factors which make for an Efficient Circulation,' by Pro- 

 fessor E. A. Schafer, F.R.S., British Medical Journal, October 29, 1910. 



