CIRCULATORY SYSTEM 779 



THE VASCULAR SYSTEM 



After food has been taken into the digestive tract and digested and 

 the little villi of the small intestines have absorbed the semi-liquid food, 

 this newly absorbed food is ready to become a part of the blood. An 

 elaborate system of blood vessels with a wonderfully intricate and elab- 

 orate pumping apparatus the heart carries this nourishment to every 

 part of the body. 



Before taking up the development of this system, known variously 

 as circulatory or vascular, it is necessary that the student understand 

 quite thoroughly what the adult organs are like and what their function 

 is. Only then may one validly attempt to ascertain how and why the or- 

 gans are placed where they are and how and why the function is what it 

 is. The central part of the vascular system is the heart. In the mammal 

 this consists of four definite chambers two auricles at the broad end of 

 the heart, and two ventricles toward the lower or apex region. The 

 structure of the heart itself is muscular. The compartments of the heart 

 and the work they do belong to the circulatory system proper and will 

 be described here. 



Every blood vessel leaving the heart, no matter whether it carries 

 arterial or venous blood, is called an artery, and every blood vessel en- 

 tering the heart is called a vein. This distinction must be kept very 

 clear. 



Then, too, it must never be forgotten that blood entering the heart 

 through a vein always enters a sinus or auricle. This auricle acts as 

 a reception-chamber for all blood entering the organ. After the blood 

 has entered this chamber it passes downward through an opening into 

 one of the ventricles, and it is from the ventricle that the blood leaves 

 the heart. 



In the higher forms of mammals, such as man (Fig. 445), blood en- 

 ters the right auricle through the large venae cavae, then passing down- 

 ward through the auricular-ventricular opening into the right ventricle. 

 From here it passes through the pulmonary artery to the lungs to be 

 aerated (that is, to be thoroughly mixed w r ith oxygen and to lose the 

 carbon dioxide that it has gathered in draining the entire body). After 

 being aerated, the blood passes- back to the left side of the heart through 

 the pulmonary vein to enter the left auricle, and then passes down 

 through a left auricular-ventricular opening into the ventricle from 

 which it sends forth the blood stream through the aorta to all parts of 

 the body, supplying the parts with food and nourishment. 



The system just described is known as the systemic because the 

 blood which leaves the heart through the aorta nourishes all parts of 

 the body. 



The arteries break up into smaller arterioles and capillaries. The 

 liquid part of the blood is called blood-plasma as long as it is contained 



