l68 THE BLOOD-PRESSURE IN THE CAPILLARIES. 



and depressions. Hiirthle's kymograph, however, furnishes sufficiently accurate 

 pictures of the pressure-variations in the arteries : these resemble sphygmographic 

 tracings. Hence, the sphygmographic pulse-tracing is at the same time a faithful 

 expression of the pulsatory variations in blood-pressure. 



Muscular exertion increases the blood-pressure. At the beginning 

 of a muscular contraction the pressure sometimes undergoes a tempo- 

 rary fall. 



When the heart's action is interrupted by continuous stimulation 

 of the vagus or a high positive respiratory pressure, the blood-pressure 

 diminishes enormously in the arteries; while, on the other hand, it 

 increases in the venous trunks because the blood flows from the arteries 

 into the veins in order to equalize the difference in pressure. This experi- 

 ment shows that when the difference in pressure is (almost) abolished, 

 the resting blood continues to exert some pressure on the blood-vessel 

 walls ; that is, in consequence of distention with blood, even in the resting 

 state, a lower pressure is exerted on the walls. 



Pathological. In man it has been found that the blood-pressure, as determined 

 by v. Basch's method, is increased in association with chronic inflammation of 

 the kidneys, arteriosclerosis, lead-colic, after injections of ergotin, and in cases 

 of cardiac hypertrophy with dilatation. It is diminished in the presence of cardiac 

 insufficiency. Digitalis often raises the blood-pressure in cases of cardiac disease ; 

 after the injection of morphin the pressure falls. During fever the blood-pressure 

 usually falls, as the shape of the pulse-curves also indicates; in cases of cardiac 

 insufficiency, chlorosis and pulmonary tuberculosis the blood-pressure is also low. 

 If the pressure falls to about 75 mm. in cases of diphtheria (children) , the prognosis 

 is grave. 



THE BLOOD-PRESSURE IN THE CAPILLARIES. 



Method. Owing to the minute diameter of the capillaries the pressure within 

 these vessels cannot be determined directly. By applying a small glass disc of 

 known dimensions to the vascular substratum and weighting it in a suitable 

 manner until the capillaries become pale, the degree of pressure that just over- 

 comes the pressure within the capillary region is determined approximately. The 

 calculation is made as follows : The pressure (expressed in centimeters of a column 

 of water) is obtained by dividing the number that represents the compressing 

 weight (weight + the weight of the glass disc) by the number of square centi- 

 meters contained in the surface pressed upon. In the capillaries of the finger, 

 when the hand is held up, this pressure is 24 mm. of mercury, and with the hand 

 dependent, 62 mm. ; in the ear it is 20 mm. ; in the gums of the rabbit 32 mm. 



Roy and Graham Brown press the vascular area to be examined from below 

 against a rigid glass disc by means of an elastic bladder provided with a manom- 

 eter; the microscope can then be focused on the glass disc. 



The tension of the blood in the capillaries of a circumscribed area 

 is increased by: (i) Dilatation of the small arteries supplying the area. 

 If the latter are dilated, the blood-pressure can be propagated from the 

 large trunks with less loss. (2) Increase of pressure in the small arteries 

 supplying the area. (3) Constriction of the veins draining the capil- 

 lary area. Occlusion of the veins causes a fourfold increase in the 

 pressure. (4) Increased pressure in the veins, as, for example, by 

 change of position (hydrostatic pressure). Diminution of the blood- 

 pressure in the capillaries is brought about by the opposite conditions. 



Also, changes in the diameter of the capillaries must have some influence on 

 the internal pressure. The inherent power of movement (movement of the proto- 

 plasm) of the capillary cells, as well as the pressure, swelling, and consistency of 

 the surrounding body-tissues must be considered in this connection. As the 



