I.— PHYSIOLOGY 175 



mined by the degree of activity of the cardio- inhibitory reflexes : indeed, 

 it has been recently shown in Belgium that the capability of dogs to 

 withstand sustained activity is apparently enhanced by removal of the 

 sympathetic. The extent to which the animals have then to rely on the 

 reduction of vagus activity is thereby increased. This of course does 

 not mean that the maximum effort for short periods is increased. To show 

 this it is necessary to time the running of the animal over short distances. 

 It has been shown that in athletes during mild exercise the cardiac 

 output is increased with a trivial increase in cardiac rate — that is, the increase 

 is chiefly produced by an increased output per beat. I shall, however, 

 return to this point. Meantime I should like to leave you with the 

 question : The heart increases its output ; where does it get its blood ? 



Experimentally it can be demonstrated that the vagus restraint of the 

 heart is extremely variable — not only in different animals, but in the same 

 animal under different conditions, as may be seen if we block the vagi. 

 For example, if we give an animal nitrogen to breathe, the normal vagus 

 restraint can be shown to have disappeared. Or we can increase the re- 

 straint by previous sensory stimulation. This last experiment is of special 

 interest, as it may give a clue as to how the normal vagus restraint is 

 built up. We know that animals and human beings which take large 

 amounts of exercise have slow hearts. How exactly this slow heart is 

 produced is not yet clear. All we can say at the moment is that certain 

 procedures such as sensory stimulation or asphyxia increase the heart 

 rate, partly by reducing vagus activity, but that subsequently this reduc- 

 tion is followed by an increase in the activity of the vagus. I would 

 indeed be glad if anyone could make any suggestions on this point. 



Variations in the Calibre of the Blood Vessels. 



As I have said, it may be taken as a general principle that in physical 

 exercise the blood is distributed to the active tissues at the expense of 

 the less active tissues. This local dilatation of vessels, combined with a 

 rise in the general blood pressure which is the result of increased cardiac 

 output and constriction of vessels in less active tissues, results in an enor- 

 mous increase in blood flow through the active muscles. This increase has 

 been measured for the vessels of the lower lip of the horse, and may be 

 demonstrated in an anaesthetised animal. The dilatation is brought about 

 by chemical and nervous means, and on this point an enormous amount 

 of work has been carried out in recent years. 



The cause of the chemical dilatation has been a matter of considerable 

 debate. It has been demonstrated that blood issuing from tetanised 

 limbs has a vasodilator action. There are first to be considered the 

 products of carbohydrate metabolism — carbon dioxide and lactic acid. 

 Each of these has been observed to cause vasodilatation if applied in suitable 

 concentrations to capillaries under the microscope. I emphasise the con- 

 centration because larger concentrations have the opposite effect. It may 

 be demonstrated also that, if the vessels of the hind limb of a chloralosed 

 animal are perfused with the nerves intact and carbon dioxide is adminis- 

 tered, the perfused vessels constrict because of the action of the carbon 

 dioxide on the vasomotor centre, but the blood-pressure does not neces- 



