CIRCULATION.] PHYSIOLOGY, 6i 



you find yourself at last in the capillaries of the lungs. 

 Squeezing your way through these, you come out into 

 veins, and gradually advancing through larger and 

 larger veins, you, still following the arrow, find your- 

 self in one of four large veins (only one of them 

 is represented in the diagram) which land you in 

 the left auricle. From the left auricle it is but a 

 jump into the left ventricle. From the left ventricle 

 the way is open, as indicated by the arrow, into the 

 tube marked Ao. This is intended to represent the' 

 aorta, which you have already seen in the sheep's 

 heart (Fig. 5, Ad), It is here drawn for simplicity'^ 

 sake as dividing into two branches, but you have 

 already been told, and must bear in mind, that it does 

 not in reality divide in this way, but gives off a good 

 many branches of various sizes. However, taking 

 the figure as it stands, suppose we travel along A^, 

 Following the arrow, and shooting through arteries 

 which continually get smaller and smaller, we come 

 at last to capillaries somewhere, in the skin or in some 

 muscle, or in a bone, or in the brain, or almost any- 

 where, in fact, in the upper part of the body. Oul 

 of the capillaries we pass into veins, which, joining 

 together and so forming larger and larger trunks, 

 bring us at last to the point from which we started, 

 the superior vena cava, V.C.S, If we had taker 

 the other road, A"", we should have passed through 

 capillaries somewhere in the lower part of the bod> 

 instead of the upper, and come back by the vens 

 cava ii. ' rior, F.C/., instead of the vena cava supe 

 rior. Starting from the right auricle, which 

 ever way we took we should always come 

 back to the right auricle again, and in out 



