1 1 2 THE MECHANISM OF THE CIRCULA TION. 



arterial pressure is very high, katacrotic when the pressure is low. The 

 change of form, as the arterial tension rises, is seen very well in the six 

 curves taken with a spring manometer during the escape of the heart 

 from vagal inhibition (Fig. 76). By compression of the abdominal 

 aorta, the carotid pulse can easily be made to exhibit an anacrotic 

 wave. 



To sum up, then — 1. The form of the pulse curve in any artery is 

 produced by (1) the aortic centrifugal primary waves ; (2) the centri- 

 petal waves reflected from the peripheral field of the observed artery ; 

 (3) the waves reflected from other arteries. These waves modify each 

 other by interference. The sphygmograms from different arteries show 

 certain main and constant features, which in each artery are modified 

 by peculiar local characteristics. For example, in femoral tracings, 

 the primary and dicrotic waves are much more prolonged than in radial 

 curves. 



2. Since the form of each pulse curve depends not only upon the 

 systole of the heart, but also on the peripheral resistance, the tone of the 

 arteries, the turgescence of the vena? comites, and the amplitude of the 

 reflected waves, it is obvious that no sure deductions can be drawn from 

 the amplitude of the oscillation, either as to the power of the heart or 

 the height of the arterial tension. 



Finally, it must be remembered that the sphygmograph can never 

 be applied twice to the same artery in exactly the same way, and with 

 the same amount of pressure. This vitiates the instrument for com- 

 parative work. The best rule to follow is to always so adjust the 

 instrument as to obtain the maximal excursion that can be possibly 

 obtained in any given case. 



THE CAPILLAEY CIRCULATION. 



The blood is brought into intimate relation with the tissues by 

 diffusing through the endothelial wall of the capillaries, and this wall 

 is of the greatest tenuity ; thereby takes place that exchange of material 

 which maintains the combustion of the body and the fire of life. The 

 whole aim of the circulation is attained, so long as the arterialized 

 blood is driven with a constant velocity through the capillaries. 



The capillaries vary in size in different organs. In the brain the 

 length has been estimated to be O709 mm. (pons), and 042 mm. (optic 

 thalamus), in the mucosa of the stomach 06 mm., and in the liver 0"5 

 to l'l mm. 1 The diameter of the capillaries varies from - 007 to - 013 

 mm. 



Malpighi 2 (1661) first observed under the microscope the capillary circu- 

 lation. He examined the lung, the mesentery, and the bladder of the frog. 

 It has since been seen in many other transparent or translucent parts of 

 animals. 



The microscopical examination of the circulation. — By using a 

 low power it is possible to simultaneously examine arteries, capillaries, 

 and veins in the same field. The first thing which strikes the 

 observer is the different direction of the stream in the arteries and in 



1 Lewy, Arch./, d. ges. Physiol., Bonn, 1896, S. 461. 



2 " De pulmonibus epistola," ii., Opera omnia, London, 1686. 



