128 BLOOD-PRESSURE IN THE CAPILLARIES. 



blood-pressure may be increased either by the action of the heart or the arterioles. 

 If we divide the vagi the pulse beats more quickly, and in some animals the 

 blood-pressure rises ; in this case, the rise in the two curves occurs together, and if 

 the vagi be stimulated there is a sudden fall of the blood-pressure, due to arrest 

 of the heart's action, so that again the two curves are parallel. If the arterioles 

 contract the blood-pressure rises, but by and by the pulse-rate falls, owing to the 

 cardio-inhibitory action of the vagus ; while, on the other hand, if the arterioles 

 are dilated, the blood-pressure falls, and the heart beats faster. Thus, in both of 

 these cases the pulse-curve and blood-pressure curve run in opposite directions. 

 These results only obtain when the vagi are intact (BrutUon).'] 



[The increase in the pulse-rate and blood-pressure following section of the vagi do not run 



Earallel. Both sooner or later reach a maximum, but the blood-pressure gradually falls to or 

 elow the normal, while the pulse-rate remains above the normal (Munzel).] 

 For the effects of the nervous system upon the blood-pressure see 871. 

 Pathological. In persons suffering from granular or contracted kidney and sclerosis of the 

 arteries, in lead poisoning, and after the injection of ergotin, which causes contraction of the 

 small arteries, it is found, on employing the method of v. Basch, that the blood-pressure is 

 raised. It is also increased in cases of cardiac hypertrophy with dilatation, and by digitalis in 

 cardiac affections, while it falls after the injection of morphia. The blood-pressure falls in 

 fever, a fact also indicated in the sphygmogram ( 69), and it is low in chlorosis and phthisis. 



86- BLOOD-PRESSURE IN THE CAPILLARIES. Methods. Direct estimation of the capil- 

 lary pressure is not possible on account of the smallness of the capillary tubes. If a glass plate of 

 known dimensions be placed on a portion of the skin rich in blood-vessels, and if it be weighted 



until the capillaries become pale, we obtain approximately the pressure neces- 

 sary to overcome the capillary pressure. N. v. Kries placed a small glass 

 plate (fig. 109) 2*5-5 sq. mm., on a suitable part of the skin, e.g., the skin 

 at the root of the nail on the terminal phalanx, or on the ear in man, and 

 on the gum in rabbits. Into a scale-pan attached to this, weights were 



f)laced until the skin became pale. The pressure in the capillaries of the 

 land, when the hand is raised, Kries found to be 24 mm. Hg. ; when the 

 hand hangs down, 54 mm. Hg. : in the ear, 20 mm., and in the gum of a 

 rabbit, 32 mm. 



Roy and Graham Brown compressed from below transparent vascular mem- 

 branes against a glass plate by means of an elastic bag connected with a 

 manometer, while the variations in the capillaries were observed from above 

 by a microscope. 



Conditions influencing Capillary Pressure. The capillary 

 blood-pressure in a given area increases (1) When the afferent 

 small arteries dilate, so that the blood-pressure within the large 

 arteries is propagated more easily into them. (2) By increasing 

 the pressure in the small afferent arteries. (3) By narrowing the 

 diameter of the veins leading from the capillary area. Closure of 

 Fig. 109. the veins may quadruple the pressure. (4) By increasing the 

 \ . Knes'a lappara- pressure in the veins (e.g., by altering the position of a limb). A 

 pressured **! d^inution of the capillary pressure is caused by the opposite con- 

 square of glass.' ditions. 



Changes in the diameter of the capillaries influence the internal pres- 

 sure. We have to consider the movements of the capillary wall itself as well as the pressure, 

 swelling, and consistence of the surrounding tissues. The resistance to the blood-stream is 

 greatest in the capillary area, and it is evident that the blood in a long capillary must exert 

 more pressure at the commencement than at the end of the capillary ; in the middle of the 

 capillary area the blood-pressure is just about one-half of the pressure within the large arteries 

 (Donders). The capillary pressure must also vary in different regions of the body. Thus, the 

 pressure within the intestinal capillaries, in those constituting the glomeruli of the kidney, 

 and in those of lower limbs when the person is in the erect posture, must be greater than in 

 other regions, depending in the former cases partly upon the double resistance caused by two 

 sets of capillaries, and in the latter case partly on purely hydrostatic causes. 



87- Blood-Pressure in the Veins. In the large venous trunks near the 

 heart (innominate, subclavian, jugular) a mean negative pressure of about -0*1 



