134 



PRINCIPLES OP VETERINARY SCIENCE 



The velocity of blood-flow should not be confused with pres- 

 sure. In order that velocity be reduced, the path must be wid- 

 ened, while reduction in pressure means resistance overcome. 

 Velocity may increase by the contraction of the channels, but 

 pressure never is increased if the flow is in horizontal vessels; it 

 can only decrease. The rate of blood flow is highest in the aorta, 

 lower in the large venae cavse, and very slow in 

 the capillaries. It may take a full second for a 

 given corpuscle to pass through a capillary 3^5 

 inch in length. In the aorta the velocity is 

 300 times faster. 



The explanation of the velocity fall from a 

 maximum in the aorta to a minimum in the 

 capillaries followed by an increase in the veins 

 is not hard to understand (Fig. 43). It is 

 merely that in any stream the velocity is greatest 

 where the cross-section of the channel is least 

 J) C and lowest where it is greatest. This may not 



be apparent unless it is recalled that the sectional 

 area of the capiUaries is several hundred times 

 that of the aorta. It has been found that in 

 the horse it takes about half a minute for a 

 substance in solution to pass from the jugular 

 vein down through the right side of the heart, 

 the lungs, the left side of the heart, up through 

 the arteries of the neck and head, and so back- 

 to the jugulsw vein where the substance was 

 introduced. The greater portion of this half 

 minute is taken up during the passage through 

 the capillaries. 



The movement of lymph is due to the same 

 agencies responsible for the flow of blood in the veins. There are 

 valves in the lymph channels which permit the fluid to advance 

 toward the heart but forbid a return. 



Good circulation is possible only when the animal has oppor- 

 tunity for exercise. When exercise is provided, breathing is 

 deeper and the skeletal muscles are rapidly contracted and re- 

 laxed. This causes the blood to be pumped out of the veins and 

 the lymph to flow freely in the lymph channels. 



Fig. 43.— If a 

 vessel (a) divides 

 into two branches, 

 (6) and (c), these 

 will be individu- 

 ally of less cross- 

 section than the 

 main trunk but 

 unitedly they will 

 exceed it. Linear 

 velocity will be 

 lower in the 

 branches than in 

 the parent stock. 

 (Stiles, Human 

 Physiology.) 



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