120 THE FLOW IN THE ARTERIES. [BOOK i. 



FIG. 17. APPARATUS FOR INVESTIGATING BLOOD-PRESSURE. 



At the upper right-hand corner, is seen, on an enlarged scale, the carotid artery, 

 clamped by the forceps bd, with the vagus nerve v lying by its side. The artery 

 has been ligatured at V and the glass cannula c has been introduced into the artery 

 between the ligature I' and the forceps bd, and secured in position by the ligature I. 

 The shrunken artery on the distal side of the cannula is seen at ca'. 



p.b. is a box containing a bottle holding a saturated solution of sodium car- 

 bonate or a solution of sodium bicarbonate of sp. gr. 1083, and capable of being 

 raised or lowered at pleasure. The solution flows by the tube p.t. regulated by the 

 clamp c" into the tube t. A syringe, with a stop-cock, may be substituted for the 

 bottle, and attached at c". This indeed is in many respects a more convenient plan. 

 The tube t is connected with the leaden tube i, and the stopcock c with the mano- 

 meter, of which ra is the descending and ra' the ascending limb, and s the support. 

 The mercury in the ascending limb bears on its surface the float fl, a long rod 

 attached to which is fitted with the pen p, writing on the recording surface r. The 

 clamp cl. at the end of the tube t has an arrangement shewn on a larger scale 

 at the right hand upper corner. 



The descending tube m of the manometer, and the tube t being completely filled 

 along its whole length with fluid to the exclusion of all air, the cannula c is filled 

 with fluid, slipped into the open end of the thick- walled india-rubber tube i, until it 

 meets the tube t (whose position within the india-rubber tube is shewn by the dotted 

 lines), and is then securely fixed in this position by the clamp cl. 



The stopcocks c and c" are now opened, and the pressure-bottle raised or fluid 

 driven in by the syringe until the mercury hi the manometer is raised to the 

 required height. The clamp c" is then closed and the forceps bd removed from the 

 artery. The pressure of the blood in the carotid ca. is in consequence brought to 

 bear through t upon the mercury in the manometer. 



addition in the minute arteries and capillaries. We may speak of 

 it therefore as the 'peripheral friction 5 and the resistance which 

 it offers as the 'peripheral resistance.' It need perhaps hardly 

 be said that this peripheral friction not only opposes the flow of 

 blood through the capillaries themselves, but, working backwards 

 along the whole arterial system, has to be met by the heart at 

 each systole of the ventricle. 



It is well known that when any portion of the skin is pressed 

 upon, it becomes pale and bloodless; this is due to the pressure 

 driving the blood out of the capillaries and minute vessels and 

 preventing any fresh blood entering into them. By carefully 

 investigating the amount of pressure necessary to prevent the 

 blood entering the capillaries and minute arteries of the web of the 

 frog's foot, or of the skin beneath the nail in man or elsewhere, the 

 internal pressure which the blood is exercising on the walls of the 

 capillaries and minute arteries and veins may be approximately 

 determined. In the frog's web this has been found to be equal to 

 about 7 or 11 mm. mercury. 



2. The Flow in the Arteries. 



When an artery is severed, the flow from the proximal section 

 is not equable, but comes in jets, which correspond to the heart- 



