232 HUMAN PHYSIOLOGY 



unusual pressure inside a vein will round it up and 

 greatly add to its capacity. It has been said that the 

 capillaries are composed of one layer of the thinnest 

 kind of epithelium; this same tissue is continued as a 

 lining through the arteries, the veins, and the cavities of 

 the heart. If all the connective and muscular tissue 

 could be removed from the circulatory system it would 

 be quite completely mapped in epithelium of inconceiv- 

 able delicacy. 



The Velocity of Blood-flow in Vessels of Different 

 Classes. Before we analyze the action of the heart we 

 will consider the main facts about the progress of the 

 blood through the vessels. The left ventricle thrusts it 

 first into the great main artery, the aorta. This rises 

 above the heart, arches over, giving off branches to the 

 head, arms, and chest, and then sweeps down in front of 

 the spinal column. It pierces the diaphragm and supplies 

 the abdominal organs, the trunk muscles at this level, 

 and finally the legs. The blood advances along the 

 aorta at a high velocity. In one second a corpuscle may 

 move from the root of the vessel to a point as much as 12 

 inches away. In all the large arteries the blood moves 

 rapidly, but nowhere so fast as at the very beginning. 

 The student is apt to leap to the conclusion that the 

 aortic velocity is maximal because the blood is there 

 under the unspent impetus of the heart's contraction. 

 This is a wrong notion as will be shown. 



The rate of flow in the capillaries can be observed 

 directly with the microscope in various transparent 

 structures, such as the mesentery. It is very slow. A 

 corpuscle is likely to take fully a second to traverse a 

 capillary ^ 5 inch in length. Such a velocity is not 

 over -3^0 of that in the aorta. It has been set as low as 

 g-J-Q. In the veins we find the blood speeding up and at- 

 taining a rate which is not much less than that in the ar- 

 teries. The highest speed anywhere in the systemic veins 

 is probably in the two large veins which lead nearly all the 

 blood into the right auricle. These are the vence cavce, the 



