THE VASCULAE SYSTEMS. 



rPHE vascular systems comprise the heart and bloodvessels (blood-vascular 

 J_ system) for the circulation of the blood, and the lymphatics and lacteals 

 (lymph-vascular system), collecting the lymph from the tissues and the chyle 

 from the digestive tract and conveying them to the great veins. 



Pulmonary Capillaries 



THE HEART AND BLOODVESSELS. 



The heart is the central organ of the blood-vascular system, and consists of 

 a hollow mass of muscle tissue; by its contraction the blood is pumped to all 

 parts of the body through a complicated series of tubes, termed arteries. The 

 arteries undergo enormous ramification in 

 their course throughout the body, and end 

 in very minute vessels, called arterioles, 

 which, in their turn, open into a close- 

 meshed network of microscopic vessels, 

 termed capillaries. After the blood has 

 passed through the capillaries it enters into 

 minute vessels called venules and from them 

 it is collected into a series of larger vessels, 

 called veins, by which it is again returned 

 to the heart. The passage of the blood 

 through the heart and bloodvessels consti- 

 tutes what is termed the circulation of the 

 blood, of which the following is an outline: 

 The human heart is divided by a septum 

 into right and left halves, and each half is 

 further separated into two cavities, termed 

 the auricle and the ventricle. The heart, 

 therefore, consists of four chambers, two, 

 the right auricle and right ventricle, form- 

 ing the right half, and two, the left auricle 

 and left ventricle, forming the left half. The 

 right half of the heart contains venous or 

 deoxygenated blood ; the left, arterial or pure 

 blood. From the cavity of the left ventricle 

 the aerated blood is carried into a large 

 artery, the aorta, through the numerous 

 branches of which it is distributed to all 

 parts of the body, with the exception of the 

 lungs. In its passage through the capillaries 

 of the body the blood carries to the tissues 

 the materials necessary for their growth and 

 nourishment, and at the same time receives 

 from the tissues the waste products result- 



,,. ,11- T i , FIG. 409. Diagram to show the course of the 



ing trom their metabolism. In doing SO it circulation of the blood. This diagram does not 

 i i if i , show that the liver also receives blood through 



becomes changed from arterial into venous . the hepatic artery. 



( 549 ) 



