174 SECTIONAL ADDRESSES 



dwell particularly on those parts of the subject which are as yet less generally 

 known. 



The heart is not like an ordinary pump which sucks fluid from one tube 

 and pushes it into another. The veins are so thin that any degree of suction 

 would cause them to close. The heart is filled by the pressure of the blood 

 which reaches it during the time it is relaxed, and adjusts the force of its 

 stroke to the amount of blood in it at the beginning of contraction. The 

 more blood reaching it, the more it pumps out, within limits. This is 

 made possible by the fact that the force with which the heart contracts is 

 increased if the heart muscle is stretched. 



The heart can also change its rate. In the past it has been usual to 

 describe the heart as being under two sets of controlling nerves, one the 

 sympathetic, which when stimulated makes the heart go fast, and the other 

 the vagus, which makes the heart go slower. Now we know that this is 

 only part of the story. The evidence is almost complete that the heart 

 is really under the control of two sets of reflexes which have this function. 

 The difference between these statements is that the second involves an 

 afferent pathway to the central nervous system, for the sympathetic and 

 the vagus are constantly carrying down impulses to the heart, and if they 

 are cut off the heart goes slow or fast as the case may be. In the case of 

 the sympathetic we do not know accurately as yet the exact source of the 

 afferent impulses, but the fact that stimulation of any sensory nerves 

 causes cardiac acceleration suggests that the source is stimulation from the 

 outside world. This is not necessarily conscious, for it has been shown 

 that a sound may accelerate the heart of a person who is asleep but during 

 waking hours the higher centres undoubtedly play a part in the accelera- 

 tion. I shall refer to this further in relation to the vaso-motor centre. In 

 the case of the inhibitory impulses which slow the heart the source of the 

 afferent impulses is known. These arise from certain sensitive regions 

 within the circulation itself. These are situated in the left side of the heart, 

 the arch of the aorta and the carotid sinuses, which are small dilatations at the 

 bifurcation of the common carotid artery in the neck. We know these 

 facts because section or anaesthesia of these nerves has the same effect 

 as section of the vagus side of the reflex arc, and it can be demonstrated 

 that nerve impulses which can be recorded electrically are constantly 

 passing up the nerves from these regions. The normal method of stimu- 

 lation has been shown to be the change of blood pressure in these parts of 

 the circulation at each beat of the heart. 



When exercise is taken, two changes occur : the sympathetic accelerator 

 impulses increase and in particular the vagus impulses are reduced. The 

 evidence for this rests on the effect of exercise and other procedures on the 

 heart rate before and after section of the vagi and with and without the 

 sympathetic. It has been shown, for example, that if the vagus nerves 

 have been cut the increase of the heart rate is, during the exercise, not 

 nearly so great as it was before they were cut. It is not possible for me 

 to discuss here how the change is brought about, except to say that it is 

 in part due to the action of the higher centres and to a rise of venous pressure. 

 The increased temperature of the blood and adrenaline liberated by the 

 suprarenal gland enhance the effect of the nervous changes ; but there 

 is not time to go into this in detail. 



What I do want to emphasise is that the range of acceleration is deter- 



