CHAPTER VI. 

 THE CIRCULATION OF THE BLOOD. 



General. We have seen that the composition of the blood 

 fits it for its function of carrying foodstuffs to the tissues 

 and removing the products of combustion ; but, for the blood 

 to exercise these offices, it is necessary that it be in communi- 

 cation wi'th the outside world and the tissues. The move- 

 ment it makes through its network of vessels in order to 

 carry products from the exterior to the interior and from 

 the interior to the exterior is what is meant by circulation. 



Pulmonary and Systemic Circulation. Two systems of 

 circulation are generally distinguished. The first is the pul- 

 monary, and is the circulation of the blood through the lungs 

 in order to get rid of carbon dioxide and to get a fresh sup- 

 ply of oxygen by aeration. The second is the systemic and 

 is the circulation through the great masses of body tissue in 

 order, by means of the lymph, to supply the tissues with 

 different solid, liquid, and gaseous nutritive material and 

 take from the tissues the products no longer needed but 

 which must be eliminated. These systems are also called 

 respectively the lesser and greater circulation. 



Discovery. The circulation of the blood was an unknown 

 fact up to 1628 when the discovery of its movements was 

 made and proved by Sir iWilliam Harvey, an English physi- 

 cian prominent in his time and now famous for this dis- 

 covery. 



The Circulatory Apparatus. The blood circulates through 

 a series of closed tubes known as blood-vessels, which divide 

 up, ramify, and go to all parts of the body. These vary from 

 large, macroscopic vessels to tiny, little, hair-like tubes, the 

 capillaries, which cannot be seen with the naked eye. 



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