2 14 ^ How a Complex Anifual Uses Food unit iv 



though it were one big mass of muscle. 

 Connected to this mass are numerous 

 blood vessels. They all seem to be con- 

 nected to the upper portion, but \^'hen 

 you cut the heart open and trace each 

 vessel to its origin vou will see that they 

 are connected with different chambers. 

 Some are connected with the auricles; 

 they are called veins. Veins carry blood 

 to the auricles. Each ventricle has one 

 large vessel connected to it; through 

 these vessels blood flows away from the 

 heart. Any blood vessel that carries blood 

 away from the heart is known as an 

 artery. 



You can gather from what you read 

 above that the contraction, or beat, of 

 the heart occurs in two stages: the con- 

 traction of the two auricles, followed by 

 the contraction of the two ventricles. 

 This occurs about 70 times per minute 

 and never stops throughout your life. A 

 frog's heart, which is slightly different 

 from ours in structure, shows this double 

 beat very clearly. It will be worth while 

 to dissect a frog and do Exercise 6. This 

 demonstration shows another interesting 

 thing about heart muscle, whether in us 

 or in the frog. It can contract rhythmi- 

 cally by itself without being connected 

 to the nervous system. This is not true of 

 the voluntary muscle in the arm or leg 

 or other parts of the body; nor is it true 

 of the involuntary muscle in the walls 

 of the alimentary canal. Heart muscle not 

 only acts differently from the other mus- 

 cles but looks different from voluntary 

 and involuntary muscle under the micro- 

 scope. 



William Harvey. You have probably 

 known for a long time about the beating 

 of the heart and how the blood flows 



through the arteries and veins. But it 

 took very many centuries for us to gain 

 an understanding of what seems so com- 

 monplace now. Before men knew that 

 the body is made of living cells which 

 are supplied with digested food and oxy- 

 gen by the blood, they imagined all kinds 

 of possible uses for the blood and for the 

 heart. For a long time the heart was be- 

 lieved to be the seat of intelligence. Later 

 it was supposed to add "vital spirits" to 

 the blood. The Greeks believed that the 

 arteries carried air, the veins carried 

 blood. Theories such as these had been 

 largely discarded, and studies of the 

 structure and uses of the heart had been 

 begun by the beginning of the 17th cen- 

 tury but had never been carried very far. 

 William Harvey (i 578-1 667), an Eng- 

 lish physician, after careful studies and 

 after performing many accurate experi- 

 ments, published the book which ex- 

 plained the circulation of the blood as we 

 now understand it. He showed that the 

 heart is muscular and serves as a pump. 

 He calculated that if the heart contains 

 two ounces of blood and beats sixty-five 

 times a minute, then it drives ten pounds 

 of blood out into the body in less than 

 a minute. Evidently, the same blood is 

 continually being pumped around; this 

 amount of blood could not possibly be 

 made anew in that space of time. He 

 knew, therefore, that blood which leaves 

 the heart must return to the heart. If 

 the arteries carry it away from the heart, 

 the veins must bring it back. Harvey did 

 not see the microscopic capillaries but he 

 suspected that there must be tiny blood 

 vessels, too small for him to see, through 

 which blood from the arteries flows to 

 the veins in all parts of the body. 



