TELOCITY OF BLOOD IN ARTERIES. 167 



Poiseuille, Ludwig, and others have confirmed what 

 Haller and Magendie observed, namely, that the strength 

 of the blood's impulse in the arteries is increased during 

 expiration ; in which act the chest is contracted, and the 

 ]arge vessels in consequence compressed. This point will 

 be again referred to in speaking of the movement of the 

 venous blood. 



Velocity of the Blood in the Arteries. 



The velocity of the stream of blood is greater in the 

 arteries than in any other part of the circulatory system, 

 and in them it is greatest in the neighbourhood of the 

 heart, and during the ventricular systole; the rate of 

 movement diminishing during the diastole of the ven- 

 tricles, and in the parts of the arterial system most distant 

 from the heart. From Volkmann's experiments with the 

 hsemodromometer, it may be concluded that the blood 

 moves in the large arteries near the heart at the rate of 

 about ten or twelve inches per second. Vierordt calculated 

 the rapidity of the stream at about the same rate in the 

 arteries near the heart, and at two and a quarter inches 

 per second in the arteries of the foot. 



THE CAPILLARIES. 



In all organic textures, except some parts of the corpora 

 cavernosa of the penis, and of the uterine placenta, and of 

 the spleen, the transmission of the blood from the minute 

 branches of the arteries to the minute veins is effected 

 through a network of microscopic vessels, in the meshes 

 of which the proper substance of the tissue lies (fig. 49). 

 This may be seen in all minutely injected preparations ; 

 and during life, by the aid of the microscope, in any trans- 

 parent vascular parts, such as the web of the frog's foot, 

 the tail or external branchiae of the tadpole, or the wing 

 of the bat. The structure of the capillaries is much more 

 simple than that of the arteries or veins. Their walls are 

 composed of a single layer of elongated or radiate, flat- 



