THE CAPILLAR Y BLOOD PRESSURE. 1 1 5 



illuminated ground-glass screen, held 11 to 16 cms. from the eye, and on 

 this screen he reckoned the progression of a corpuscle in a given time. 

 He found that a blood corpuscle could be thus followed over a space 

 of 20 to 30 mm. If A represents the distance of the screen from 

 the anterior nodal point, and B the distance of the retina from the 

 posterior nodal point of the eye, and C the distance travelled by the 



He 



corpuscle on the screen, then — - =x } the real distance which is covered 



by the corpuscle in the capillary. 



The mean velocity was thus found to be 0'75 mm. per second. The 

 red corpuscles travel in the axial parts of the stream, and as the mean 

 velocity is less than the axial velocity, the true mean velocity of flow 

 is less than the above ; it may be taken to be about - 5 mm. Since the 

 velocity at any point in a system of tubes stands in inverse propor- 

 tion to the sectional area, the relationship of the total sectional area 

 of the capillaries, at any one time patent, to that of the aorta can be 

 reckoned. 1 Thus, if the mean velocity be taken as 320 mm. per second 

 in the aorta and 0*5 mm. per second in the capillaries, the relation 

 is 1 : 640. In man the sectional area of the aorta is 4 - 4 sq. cms. 

 The total sectional area of the capillaries would thus be equal to 

 about 2800 sq. cms. This result is, of course, only roughly approxi- 

 mative. 



The capillary blood pressure. — The pressure was first obtained 

 by v. Kries. 2 He placed a glass plate, 2 - 5 to 5 sq. mm. in size, in a 

 suitable place on the skin, such as the last joint of the finger. There 

 depended from this glass plate a small scale pan. On this, weights were 

 placed until the pressure was reached at which the skin was blanched 

 and the capillaries compressed. 



In using this method, the reading is taken when the change of colour in 

 the skin just begins to appear, that is, when the superficial capillaries are 

 obliterated. Thus the error due to the elasticity of the epidermis is avoided 

 as far as possible. It is presupposed that the elasticity of the capillary wall is 

 negligeable, and that the blood pressure, in all the area of capillaries under 

 observation, is equally great. 



If a weight of g grammes be placed on a surface of / square milli- 

 metres, then the formula h = -- gives the hydrostatic pressure in mm. H 2 



which is required to obliterate the capillaries. Thus, suppose the square 

 surface of the glass plate be 4 sq. mm., and the weight be 1 grm., then an 

 area of 4 sq. mm. carries 1 grm. = 1 c.c. H 2 = 1000 cub. mm. The height 



of the column of water supported by one sq. mm. is therefore — - — = 250 mm. 



H,0. 



0*25 gratis, is the smallest difference in weight which can produce the index 

 of colour change. To obtain the index with a smaller glass plate, a heavier 

 weight is needed than with a larger plate. The method is therefore very 

 rough and inaccurate. 



Boy and Graham Brown 3 employed a method of directly determining 

 how great a hydrostatic pressure is necessary to obliterate the capillaries 

 while under microscopic observation. The web or mesentery of a frog 



1 Vierordt, Arch. f. physiol. Heilk., Stuttgart, 1848, S. 184. 



2 Bcr. d. k. Sachs. Gcscllsch. d. Wisscnsch., math.-phys. CI., Leipzig, 1875, S. 148. 



3 Joum. Physiol., Cambridge and London, 1879, vol. ii. p. 328. 



