260 THE MECHANICS OF THE HEART 



The second principal division of the circulatory system is fonned 

 by the lesser or pulmonary circuit. It consists of the pulmonary artery 

 and its branches which conduct the blood from the right ventricle into 

 the capillaries of the lungs, and of the pulmonary veins which collect 

 the aerated blood and return it into the left auricle. Thus, while 

 every drop of blood is forced to traverse the greater and lesser circuits 

 successively, the course which it may pursue is not restricted to one 

 and the same channel, because it may pass either into the capillaries 

 of the head or into those of the heart, portal organs and posterior ex- 

 tremities. In other words, a large number of shorter and longer 

 paths are open to it. 



In perfect agreement with the circulation in the lower forms, the 

 blood of the mammal is made to flow in the direction indicated, 

 because the contraction of the auricles antecedes that of the ventricles 

 by a definite period of time, and because the circulatory channel is 

 beset with valves which open only in one particular direction. As far 

 as the second factor is concerned, it should be stated at this time that 

 there are three sets of valves to be considered, namely: (a) the auriculo- 

 ventricular which guard the openings between the auricles and ven- 

 tricles, (6) the semilunar which are situated in the orifices of the aorta 

 and pulmonary artery, and (c) numerous venous valves which are 

 placed as a rule at the points of confluency of small and large veins. 

 The first set of valves comprises the tricuspid and mitral, the former 

 being placed in the right and the latter in the left orifice. Both open 

 downward into the cavities of the ventricles. The second set con- 

 sists of the pulmonary and aortic semilunar valves. Their flaps yield 

 outward, i.e., in a direction away from the ventricles. The venous 

 valves open only toward the heart. 



The Circulatory System During Fetal Life. — The circulatory 

 system of the adult human being finds its origin in the system which is 

 present during the last months of intra-uterine life. The complete 

 separation of the young from the mother effected at birth, necessitates 

 first of all the presence of a heart that is capable of developing an 

 adequate driving force, and secondly, several very definite alterations 

 in the distribution of certain blood-vessels which insure a perfect con- 

 tinuity of the vascular channels. It should be emphasized, however, 

 that the changes effected at birth, are not the only ones to which the 

 circulation of the human embryo is subject to. Thus, it has been 

 established that the early vitelline system which is fully developed 

 during the third week, is modified several times to meet new conditions, 

 and its shortcomings are soon compensated for by the formation 

 of the allantoic vessels which are specialized further into the placental 

 circulation. The following peculiarities are evident during the last 

 months of gestation. The blood spaces of the placenta which he in 

 contact with the enormously enlarged capillaries of the uterus, unite 

 eventually to form two blood-vessels, commonly known as the 

 umbilical artery and vein. The latter conveys the blood from the 



