THE FREQUENCY OF THE PULSE. 53 
pressure, then the heart must always recommence to beat when the 
_ arterial tension has fallen a certain proportion; for with double pres- 
_ sure and consequently double amount of blood, the time of flow 
through the capillaries is constant, as the flow varies directly as the 
pressure; and with double resistance and unvaried pressure the time 
of flow is double also, for the heart pumps again when as much has 
gone from the capillaries as it has sent into the arteries and the rela- 
tive capacities of the heart and arteries do not vary according to the 
But does the capacity of the arterial system vary as the pressure ? 
This is a point which it is very difficult to prove. With regard to 
the heart, the following facts bear on it:—By connecting a syringe 
_ with the coronary arteries, or by tying it into the aorta and pumping Page 14. 
_ backwards, it can be shown that increasing the pressure in the coro- 
nary arteries increases the capacity of the ventricles. Also in many 
post-mortem examinations the heart is found with the ventricular 
cavities fully obliterated, and as they are not then in action, the capa- 
city of the heart and the pressure in it are at a minimum together. 
This is all the direct evidence that it is in my power to bring on this 
point. 
With regard to the arterial system and its capacity, the absence of 
blood in the arteries after death has been known from time imme- 
__—«moriail, and if their capacity varied directly with the pressure, it is 
___ evident that that must be the case, both capacity and pressure being 
— ata minimum. 
A direct method of determining this point having occurred to me, 
the following description will illustrate it. In a rabbit one of the 
carotids was put in communication with a kymographion ; and during 
the time the recording drum was revolving, the chess was suddenly 
opened and the ventricles cut across transversely. The pressure fell 
rapidly to zero, and it is clear that the fall must have arisen from the 
escape of the blood through the peripheral vessels, as the aortic valves 
would close immediately. The curve of descent would take a definite 
form, which is easily expressed in mathematical language, if the 
capacity diminished as the pressure. ‘ Unfortunately the time required 
= to open the chest, and other difficulties connected with the operation, 
"prevented my results from being of much value, and Dr. Michael — 
Foster suggested to me that the same object would be attained if the Page 15. 
heart were made to stop by the action of the interrupted current on 
the pneumogastric nerve. Mr. Martin of Christ’s College, Cambridge, 
kindly sent me some traces thus taken, and one of the two which are 
suitable for measurement, entirely conforms with the law that the 
capacity of the vessels varies directly as the blood pressure, assuming 
Poiseuille’s law to be correct. The other curve does not exactly fulfil 
