

PERIPHERIC ORGANS OF THE CIRCULATION. 145 



veins, on the contrary, the pressure is found to be ex- 

 tremely feeble, as an examination of the above diagrams has 

 shown. The pressure in the capillaries cannot be measured 

 exactly : it is, probably, as we have said, ^fa of the atmos- 

 phere. In hemorrhage from the capillaries, however, the blood 

 does not come out in jets : its flow is here greatly retarded 

 by the friction which it undergoes against the walls of these 

 small tubes. If we examine the circulation of the capillaries 

 with a microscope, we shall see that all the external portions 

 of the blood current as it flows adhere to the walls of the 

 vessels, almost without motion (passive layer) ; the central 

 column alone moves, drawing with it the globular elements 

 of the blood, especially the red globules ; for 

 the white globules, which are extremely 

 viscous, are easily caught, and arrested in the 

 passive layer (Fig. 44). 



These ideas as to the distribution of pres- 

 sure in the circulatory system, though so sim- 

 ple, were not easily acquired. Poiseuille at 

 first maintained that the pressure was the 

 same at all points of the circulatory system, 

 at whatever distance from the ventricle. This 

 view, which reason alone might have shown to 

 be an error, was experimentally overthrown by 

 Marey ; who has demonstrated that in the vas- 

 cular system, from the heart to the capillaries, 

 the pressure is distributed as in a liquid placed 

 in a tube with one end open, and the other 

 communicating with the bottom of a vase filled 

 with liquid at a certain pressure. Poiseuille had also imag- 

 ined that the general pressure varied in animals of different 

 bulk, and always in proportion to their size. But Claude 

 Bernard has demonstrated that this is not at all the case since 

 the same apparatus with which we measure the mean or mini- 

 mum pressure in a rabbit is quite sufficient to measure the 

 same pressure in a horse. But, by means of the cardiometer, 

 he has also shown that two things must be distinguished in the 



employed (Poiseuille and Ludwig's instruments), this advantage, that it records 

 exactly the cardiac pulsations ; because the mercury, in this case, fills a compara- 

 tively large bottle, and not simply a tube in the shape of a U ; and the whole 

 mass of the mercury is not displaced at every change of pressure ; neither does 

 that friction take place which produces the loss of a large part of the force to be 

 measured. 



* r, Central current of the red globules. I, /, I, Peripheral layer of the blood 

 current, in which the white globules move more slowly. (280 diam.) 



U 



terdigital mem- 

 brane of a frog.* 



