186 CIRCULATION 



From this auricle the blood passes into the left ventricle 

 through an aperture similar to that between the right 

 auricle and ventricle. This opening is guarded by the 

 mitral valve, which is fastened and operates in a manner 

 similar to the tricuspid valve. From the left ventricle a 

 single large blood vessel opens, called the aorta or great 

 artery. This, Ukc the pulmonary artery, is guarded from 

 back flow at its base, by semilunar valves. Near its base, 

 also, separate two smaller arteries (the coronary arteries) 

 which convey blood to the substance of the heart itself. 



Arteries, veins, and capillaries. — (See Ex. XLIV.) In 

 the preceding paragraphs wc have used the terms artery 

 and veins to indicate blood vessels, but without distin- 

 guishing clearly between them. What is the meaning of 

 the terms artery, vein, and capillary as applied to blood 

 vessels? 



An examination of the back of the hand will show us a 

 number of dark, bluish colored lines, which give no sensa- 

 tion of beating when we place the finger upon them. 

 These lines are actually blood tubes lying underneath 

 the skin, and their color and lack of beat or pulse indicates 

 that they are veins. If now, wc place our finger on the 

 inside of the -wrist we shall find that there is a certain spot 

 where a regular beating or pulse ic felt. That spot marks 

 the presence of en artery near the surface, and if we look 

 closely we can make out a thin line less blue than the 

 surrounding veins. 



AVhile almost all parts of the body surface show blue 

 veins the pulse spots are very few, the most prominent 

 being located on the inside of the wrists, the temples, and 

 the under side of the knee. This indicates that the arter- 

 ies are less near the surface than the veins and are thus 



