CIRCULATORY SYSTEM 781 



turn, has left the left ventricle. The blood vessels in the walls of blood 

 vessels are known as vasa-vasorum. 



It is essential that these two systems be kept separate and distinct. 

 The mere pouring of blood into the cavities of the heart is equivalent 

 to the water in the tank of an hydraulic system, while the blood which 

 enters the heart muscle itself is equivalent to the water in the engine's 

 own boiler that furnishes the steam from which the energy, in turn, 

 comes that makes pumping possible. 



This analogy may be carried a little further; for, just as the water 

 in the hydraulic system, if it be used for drinking purposes, must be 

 filtered, so, before the blood, which is pumped through the vascular 

 system, can be used again, it must likewise be filtered. This is the work 

 of the portal systems. 



A final point is to be borne in mind, before taking up the circulatory 

 system in detail, is that the vertebrate circulatory system is known as a 

 closed circulation, as contradistinguished from the open system seen in 

 some of the lower forms of animals such as the crayfish. 



What is meant by a closed system, is that the blood from the time 

 it leaves the heart, until it returns, is always in direct communication 

 by means of arteries, capillaries and veins There are no open 'spaces 

 through which the blood can pass out of these vessels. A seeming ex- 

 ception is the lymph. This does not pass through an opening, however, 

 but seeps directly through the Avails and bathes all parts of the inter- 

 capillary region. 



DEVELOPMENT 



Everyone knows that the mammalian heart has its point or apex 

 to the left, but the student must know how this has come about. He 

 must also know why it is that, just as with the digestive tract, certain 

 nerves which lie in the right and left side of the early embryo, come to 

 lie on the dorsal and ventral sides in the adult. The heart, like the 

 digestive tract, grows something on the order of a straight tube, although 

 made up of separate cephalic and caudal ends which have become fused 

 together. As the embryo continues developing, the heart turns to the 

 left, so that the nerves which lie upon the right side will now be ventral 

 and those which lie upon the left side will be dorsal, while the right 

 auricle and ventricle which have been brought ventral by this turning 

 to the left now occupy almost the entire ventral portion of the heart, 

 the left auricle and ventricle being dorsal. It is for this reason that a 

 very small portion of the left auricle and ventricle can be seen from 

 the ventral side of the body. It will be remembered that in the. embryo 

 of the chick we spoke of mesoblastic cells which were derived from 

 three separate sources. One source of these is from the primitive streak. 

 The second source is from that scattered group of cells that was left 

 between the ectoblast and entoblast when the entoblast became a dis- 



