THE FLOW OF BLOOD IN THE CAPILLARIES 



221 



tudinal folds or wrinkles appear in the cell membrane, which increase in num- 

 ber, clearness and extent as the capillary wall draws together, and entirely, 

 disappear when the vessel dilates again (Fig. 90). However, certain individual 

 capillaries or capillary tracts are quite exempt from this contraction (Steinach). 



The most favorable object for the demonstration of these phenomena is the 

 nictitating membrane of the frog. Quite similar results have been obtained also 

 in other capillary systems of the frog and even of Mammals. 



Capillaries are abundantly supplied with nerves, and Steinach has succeeded 

 in producing a contraction of the capillaries in the frog's nictitating membrane 

 by stimulation of the sympathetic. 



Since the diameter of the capillaries may vary independently of the blood 

 pressure, it follows with great probability that in virtue of their contractility 

 the capillaries themselves participate to a considerable 

 extent in the regulation of the blood supply to the differ- 

 ent organs. 



The blood in the capillaries flows in the following man- 

 ner. If the capillary vessel is not so small that the blood 

 corpuscles entirely fill it, the red corpuscles move along with 

 their long diameters in the direction of the current, and 

 keep to the center of the vessel, so that between them and 

 the vascular wall a clear space is left filled with plasma. 

 In this space are found numerous white blood corpuscles 

 which sometimes come to rest there, sometimes roll along 

 very slowly making frequent pauses. As a rule, the cur- 

 rent in the capillaries is continuous. But there are excep- 

 tions to this rule. With sufficient dilatation of the small 

 arteries in a given vascular region the blood stream in the 

 capillaries may exhibit rhythmical vibrations synchronous 

 with the heart beats. A continuous flow presupposes there- 

 fore that the blood in the small arteries meets with suffi- 

 cient resistance to obliterate these pulsations. 



FIG. 91. Apparatus 

 for determining 

 the blood pressure 

 in the capillaries, 

 after Ludwig. The 

 small glass plate a 

 is placed on the 

 skin, and the pan 

 b is loaded with 

 weights until the 

 skin underneath a 

 is blanched. 



In the field of the microscope the velocity of blood 

 flow in the capillaries can be determined by simply ob- 

 serving the time consumed by a particular corpuscle in 

 traversing a measured distance on the eyepiece microm- 

 eter. The velocity determined in this way is given as 

 0.5-0.8 mm. per second. These values however are maximal, for they relate 

 to the current in the central part of the vessel. The mean velocity is some- 

 what less. 



Attempts have been made to determine the blood pressure in the capillaries 

 by measuring the pressure upon the outer surface of the skin or upon the 

 gums of the teeth (Fig. 91), at which a distinct change in color appears. 

 This is said to indicate that the most superficial capillaries are completely 

 compressed. The limits of error of this method are rather wide, and the 

 values obtained can only be regarded as bare approximations to the truth. 

 This appears more clearly when we bear in mind that the pressure thus de- 

 termined is not the total capillary pressure, for the lymph exerts an opposite 

 pressure on the outer side of the capillary wall, and this depends upon the 

 tension and the turgor of the skin. 



The capillary pressure which one obtains when the effect of the hydrostatic 



