CHAPTER VI. 

 THE CIRCULATION OF THE BLOOD. 



General. We have seen that the composition of the blood fits 

 it for its function of carrying food stuffs to the tissues and remov- 

 ing the products of combustion; but, for the blood to exercise 

 these offices, it is necessary that it be in communication with the 

 outside world and the tissues. The movement 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 supply 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 William Harvey, an English physician promi- 

 nent in his time and now famous for this discovery. 



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 



