8o 



A MANUAL OF VETERINARY PHYSIOLOGY 



A relationship exists between the heart-rate and the condition 

 of blood-pressure ; when the blood-pressure becomes low, the 

 heart-rate increases as the result of reflex stimulation, by which 

 means the output of blood is increased. If the temperature of 

 the blood be raised, the heart-beat increases in frequency, and 

 there appears but little doubt that one cause of the increased 

 pulse-rate in fevers is the actual temperature of the circulating 

 blood. If the temperature of the blood be raised experimentally, 

 it is found that a point is reached at which the heart ceases to 



beat ; in the cat this has been found 

 to be between 44 to 45 C. (ni° to 



ii3° F.). 



The Velocity of the Blood varies 

 in the arteries, capillaries, and veins, 

 being greatest in the former, least 

 in the capillaries, and rising again in 

 the veins. 



The velocity of flow is inversely 

 as the sectional area of the tubes ; 

 the total sectional area of the capil- 

 laries is greater than that of the 

 aorta, therefore the velocity is re- 

 duced ; from the capillaries to the 

 heart the area becomes smaller and 

 the velocity increases. The velocity 

 of blood-flow depends on the width 

 of the bed formed by the vessels ; 

 as the arterial system expands the 

 velocity diminishes ; in passing 

 through the capillaries with their 

 immense network the velocity is at 

 a minimum ; in passing towards the 

 heart the vessels are reduced in 

 number, hence the bed is smaller, 

 and the velocity accordingly in- 

 creased. The cause of the flow 

 throughout the entire system is the 

 contraction of the left ventricle, and the gradual fall in pressure 

 which occurs from the aorta to the right auricle. 



The vascular system has been compared to two cones placed 

 base to base, the apex of one being the left ventricle, of the other 

 the right auricle ; where the bases of the two cones meet is the 

 capillary network. The sectional area of this has been estimated 

 by Volkmann as 800 times greater than that of the aorta, while 

 owing to the width of the bed the passage of blood through it is 

 800 times slower than in the aorta. 



Fig. 36. — Stromuhr of Ludwig 

 and dogiel 



A, B, Glass bulbs ; a, a metal 

 disc, to which C and D are 

 attached, and which can be 

 rotated on the disc b ; E and 

 F, cannulae attached to b, 

 and connected with the periph- 

 eral and central ends of a 

 divided bloodvessel. 



