122 ARTERIAL PRESSURE, [BOOK i. 



will be discussed hereafter. This observation teaches us that the 

 blood, as it is passing along the carotid artery, is capable of support- 

 ing a column of mercury of a certain height (measured by the 

 difference of level between the mercury in the descending limb, 

 and that in the ascending limb, of the manometer), when the 

 mercury is placed in direct communication with the side of the 

 stream of blood. In other words, the blood, as it passes through 

 the artery, exerts a lateral pressure on the sides of the artery, equal 

 to so many millimeters of mercury. In this lateral pressure we 

 have further to distinguish between the slighter oscillations corre- 

 sponding with the heart-beats, and a mean pressure above and 

 below which the oscillations range. A similar mean pressure with 

 similar oscillations is found, when any artery of the body is 

 examined in the same way. In all arteries the blood exerts a 

 certain pressure on the walls of the vessels which contain it. This 

 is generally spoken of as arterial pressure or arterial tension, and the 

 pressure in the aorta of any animal is usually spoken of as its blood- 

 pressure. 



Description Of Experiment. The carotid, or other vessel, is laid 

 bare, clamped in two places and divided between the clamps. Into the 

 cut ends is inserted a hollow | piece of the same bore as the artery, the 

 cross portion forming the continuation of the artery. The other portion 

 is connected by means of a non-elastic flexible tube with the descending 

 limb of the manometer. In order to avoid loss of blood, fluid is in- 

 jected into the flexible tube until the mercury in the manometer stands 

 a very little below what may be beforehand guessed at as the probable 

 mean pressure. The fluid chosen is a saturated solution of sodium car- 

 bonate or a solution of sodium bicarbonate of sp. gr. 1083, with a view to 

 hinder the coagulation of the blood in the tube. When the clamps are 

 removed from the artery the blood rushes through the cross of the f piece. 

 Some passes into the side limb of the { piece and continues to do so 

 until the mean pressure is quite reached. Thenceforward there is no more 

 escape ; but the pressure continues in the interior of the | piece, is 

 transmitted along the connecting tube to the manometer, and the mercury 

 continues to stand at a height indicative of the mean pressure with 

 oscillations corresponding to the heart's beats. Practically the use of 

 the | piece is found inconvenient. Accordingly the general custom is 

 to ligature the artery, to place a clamp on the vessel on the proximal 

 side of the ligature, and to introduce a straight cannula, Fig. 17 c, 

 connected with the manometer, into the artery between the ligature and 

 the clamp, and to secure it in that position. In this case, on loosing the 

 clamp, the whole column of blood in the artery is brought to bear on 

 the manometer, and the tracings taken illustrate the lateral pressure 

 not of the artery in which the cannula has been placed, hut of the vessel 

 (aorta &c. as the case may be) of which it is itself a branch. 



Tracings of the movements of the column of mercury in the mano- 

 meter may be taken either on a smoked surface of a revolving cylinder 

 (Fig. 1), or by means of a brush and ink on a continuous roll of paper, 

 as iii the more complex kymograph (Fig. 19). 



