180 EFFECTS OF THE POSITION OF A PART 
in the diameter of an artery in systole, it is inappreciable by ordinary methods 
of measurement. 
Yet, the increase produced in the pressure of the blood upon the arterial 
walls by the ventricular contractions is certainly very great. When the coronary 
artery of the lip is divided in a child in the operation for hare-lip, the little 
fountain of blood that springs from the cut vessel may be sometimes seen to 
rise about twice as high in systole as in diastole, implying that even in so small 
a branch, in spite of the equalizing tendency of the elasticity of the tubes of 
transmission, the systolic pressure of the blood is double the diastolic. 
The tracings given by the recording haemodynamometer, and reproduced 
in modern physiological works to illustrate the variations of the blood-pressure, 
though very interesting in some respects, are entirely untrustworthy as indi- 
cations of the relative pressures of systole and diastole. For, in all such 
instruments, the apparatus opposes more or less resistance to the altering pres- 
sure, and time is required to overcome that resistance, so that rapid variations, 
such as those of the cardiac pulsations, are most inadequately represented. 
With reference to the present inquiry, I was anxious to obtain definite 
information on this important point ; and it occurred to me that this might be 
simply and surely done by making the blood write its own record, by means of 
a stream issuing from a minute orifice in a tube tied into an artery, the projected 
blood being allowed to fall upon a horizontal sheet of paper drawn smoothly 
past the animal. By such an arrangement, the effects of the varying degrees 
of pressure of the blood would be observed untrammelled by any resisting 
apparatus ; and, as the range of projection is directly proportioned to the 
projecting force, a comparison of the distance to which the blood was thrown 
not obtain evidence of transverse yielding by measurement with the compasses, but only by calculation 
from the observed increase in length which the vessel experienced when over-distended with liquid, 
together with the increase in the volume of the tube, as indicated by the additional weight of water 
which it admitted, vide op. cit., pp. 407, 422, and 423. The impressions conveyed to the finger in 
feeling the pulse, and also the indications of the sphygmograph, are no evidence of expansion in the 
cylindrical tube. In both cases, pressure is made upon it, producing a more or less oval condition of 
the transverse section of the vessel; and the impulse experienced is essentially the result of a tendency 
to restoration to the circular form as the blood-waves pass through the constricted part. A precisely 
similar pulse is produced in a cylindrical tube of inelastic but flexible material, through which a fluid 
is forcibly driven in a jerking stream. In an actual experiment made to illustrate this point, the in- 
elastic tube was formed of a strip of thin macintosh cloth, with its edges stitched closely together. 
One end of this tube was connected with a piece of caoutchouc tubing, through which water was driven 
by a force-pump, while the other end of the inelastic tube was continued in another piece of caoutchouc 
tubing, the distal end of which had a somewhat narrow glass tube tied into it to simulate the resistance 
in the capillaries, and ensure a continuous though jerking stream by bringing into play the elasticity 
of the caoutchouc and of the air in the air-chamber of the pump. At every stroke of the pump, the finger, 
placed on the inelastic tube of macintosh, experienced a sensation exactly similar to that in feeling 
the pulse. When the pulse is visible in the human subject, the appearance is unquestionably caused 
by the movements of the vessel, as it becomes alternately curved and straightened in systole and diastole. 
