224 CIRCULATION OF THE BLOOD. 



quency. (See Fig. 206.) When extremely frequent, they are 

 mere undulations; but when the intervals are longer, they ex- 

 hibit forms which, as we shall afterwards see, have a definite 

 relation to the changes of tension which actually occur in the 

 arteries during each cardiac period. It is further seen that 

 there are larger waves which correspond, not to the beats of the 

 heart, but to the respiration the valley and ascending limb 

 of each of these greater undulations corresponding to inspira- 

 tion, the summit and descending limb to expiration and to the 

 pause. These and other details will be referred to in future 

 sections. 



SECTION II. OBSERVATION OF THE SUCCESSIVE CHANGES OF ARTE- 

 RIAL TENSION WHICH OCCUR DURING EACH CARDIAC PERIOD. 



In studying tracings obtained by the mercurial kymograph, 

 it is to be borne in mind that what is inscribed on the C3 7 linder 

 is not the record of the actual movement of the artery, but of 

 the oscillations of the mercurial column. It is true that the 

 latter are the immediate results of the former, and that the 

 elevation of the distal column produced by each arterial ex- 

 pansion has some relation to the increase of lateral pressure, 

 of which the expansion is the expression ; but the curve drawn 

 is not that of the arteiy, but of the manometer. The artery 

 expands suddenly, the mercury rises comparatively slowly, so 

 that at the moment it attains its acme the artery has already 

 collapsed. Consequently, if the interval between each pulsa- 

 tion and its successor is very short, the extent of oscillation 

 (or, as it is usually called, the excursion) of the manometer 

 is relatively too small ; and conversely, if the interval is much 

 prolonged, the excursion is relatively too great. The descent 

 of the column is almost entirely independent of the collapse 

 of the artery. It falls back to equilibrium, and describes a 

 curve, which (as may be learnt by comparison) has the same 

 characters as that made by the lever in returning to its origi- 

 nal position, by whatever way as, e. (7., b}'' squeezing the con- 

 necting-tube the equilibrium of the manometer may have been 

 momentarily disturbed. 



This being the case, it is easy to understand that no conclu- 

 sion can be derived from observations with the mercurial mano- 

 meter, cither as to the duration of the effect produced by each 

 contraction of the heart, or as to the relative duration of the 

 periods of expansion and collapse. The use of the instru-. 

 ment is limited to the investigation of the mean pressure, and 

 of those varieties of pressure of which the periods of recur- 

 rence are long enough to prevent their being interfered with 

 by the proper oscillations of the instrument. 



