CIRCULATION 



351 



In the frog some of these blood vessels are somewhat difficult to find; 

 otners may easily be seen. One which collects blood from the skin and 

 muscles near the ventral surface of the body, 

 but chiefly from the hind legs, is called the 

 abdominal vein. It may be seen near the 

 surface, on the ventral midline of the body, as 

 we open the frog from the ventral side. This 

 vein turns inward at a point nearly between 

 the fore limbs, divides into two branches, 

 and enters the liver. Just before it reaches the 

 liver, another vein, bringing blood from the 

 digestive tract, joins with it. The rest of 

 the blood from the hind legs has to pass 

 through what is known as the renal portal 

 system of circulation, the veins of which send 

 the blood through the kidney, and thence, 

 by a large single vein (the postcaval vein) to 

 a thin-walled sac on the dorsal side of the 

 heart. This sac, known as the sinus venosus, 

 receives the blood from the veins and emp- 

 ties it into the heart. Immediately before 

 reaching the sinus, the blood from the liver 

 (the so-called portal circulation) joins with 

 the postcaval vein. It is seen in the portal 

 circulation that part of the blood of the 

 body passes through the liver before reaching 

 the heart. Blood from the head region is 

 returned to the heart by two large precaval 

 veins. The blood from the fore limbs also 

 takes this course. 



^2cV.' 



Venous system of the frog; a.b.d., 

 abdominal vein; f.r., femoral 

 vein; H, heart; L, lungs; K, 

 kidney; LIV., liver; P. F., pul- 

 monary vein; Pr. CV.V., pre- 

 caval vein; R., R., renal veins; 

 SV, sinus venosus; Sc.V., sci- 

 atic veins. (After Parker and 

 Haswell.) 



Circulation of the Blood in Man. — As in the frog and other 

 vertebrate animals, the organs of circulation are the heart and blood 

 vessels. These blood vessels are called arteries when they carr}^ 

 blood away from the heart, veins when they bring blood back to 

 the heart, and capillaries when they connect the arteries with the 

 veins. Except in the spleen, where the blood capillaries are open, 

 blood flowing between and around the cells, the organs of circu- 

 lation form a system of closed tubes through which the blood flows 

 in a continuous stream. 



The Heart; Position, Size, Protection. — The heart is a cone- 

 shaped muscular organ about the size of a man's fist. It is located 

 immediately above the diaphragm, and lies so that the muscular 

 apex, which points downward, moves in beating or contracting 

 against the fifth and sixth ribs, just a little to the left of the mid- 

 line of the body. This fact gives rise to the notion that the heart 

 is on the left side of the body. The heart is surrounded by a loose 



