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ELEMENTS OF BIOLOGY 



which acts to accelerate and the other to inhibit the rate of beat. 

 Two different brain centres are involved. A variety of conditions 

 may affect rate of beat by acting on one or the other of these centres. 

 Acceleration of beat normally accompanies exercise. We have seen 

 how exercise changes the chemical composition of the blood. In 

 some way this blood change acts to stimulate the nerve centre that 

 increases heart beat, and thus assures an increase in the blood supply 

 to the tissues as well as a more rapid flow through the lungs and 

 an increase in tissue oxygen supply. This accelerated beat continues 

 after the exercise has ceased, until the normal composition of the 



blood is restored. While the rate of 

 heart beat may be affected by emo- 

 tional states, it is not under voluntary 

 control. 



Blood Pressure. As the left 

 ventricle contracts it sends an amount 

 of blood equal to its capacity out into 

 the great aorta with considerable sud- 

 denness. A iseries of contractions thus 

 sets up a corresponding series of waves 

 in the arteries, each wave indicating a 

 ventricular contraction. This regular 

 sequence may be noted at regions 

 where arteries are near the body sur- 

 face and is commonly called the pulse. 

 As the blood proceeds to smaller vessels the pulse becomes less 

 marked and disappears in the capillaries, where it becomes dissipated 

 by the resistance offered by the walls and the back pressure of blood 

 already filling the tiny vessels. 



The walls of the arteries consist largely of circular muscle fibres, 

 together with connective tissue and a lining of thin cells known as 

 an endothelium. In the vein walls the muscle elements are almost 

 or completely lacking (Fig. 125). Within the closed system of 



Fig. 124. — ^Muscle cells from 

 the wall of the human heart. 

 (After Schafer: Essentials of His- 

 tology. 1 2th ed. Longmans, Green 

 and Co.) 



