384 VETERINARY PHYSIOLOGY 



would be many times the sectional area of the aorta. And 

 if all the capillary vessels were cut across and placed 

 together, the sectional area would be about 700 times that 

 of the aorta. 



From the capillaries, the sectional area of the veins and 

 lymphatics steadily diminishes as the smaller branches join 

 with one another to form the larger veins and lymphatics ; 

 but, even at the entrance to the heart, the sectional area of 

 the returning tubes, the veins, is about twice as great as that 

 of the aorta. 



The circulatory system may thus be compared to a 

 stream which flows from a narrow deep channel, the aorta, 

 into a gradually broadening bed, the greatest breadth of the 

 channel being reached in the capillaries. From this point 

 the channel gradually narrows until the heart is reached. 



Hence the blood stream is very rapid in the arteries 

 where the channel is narrow, and very sluggish in the 

 capillaries where the channel is wide, so that in them plenty 

 of time is allowed for exchanges between the blood and the 

 tissues. 



II. THE CENTRAL PUMP— THE HEART. 



A. Structure. 



1. Myocardium.— The heart in the early embryo is a simple 

 tube which undergoes regular rhythmic contractions. These 

 start from the venous end and pass along the tube to the 

 other end, and thus force the blood from the veins to the 

 arteries. 



As development advances, a receiving chamber — the 

 auricle, and an expelling chamber — the ventricle, grow out 

 from the primitive tube, and thus break up the continuous 

 sheath of primitive tissue which constitutes the embryonic 

 heart (fig. 163). 



In fish this primitive tissue is found as a ring round the 

 entrance of the veins into the heart with an extension, as a 

 narrow band over the auricles to the ventricles (S. F.). 



In the mammal, where the heart is double, one part of 



