THE DEVELOPMENT OF THE VASCULAR SYSTEM. 231 



nally into two distinct vessels, one of which is connected with the right ventricle 

 and becomes the pulmonary artery, the other with the left ventricle and becomes 

 the proximal part of the aortic arch (Fig. 203). The result of the formation of 

 these various septa is the division of the entire heart into two sides. The 

 atrium and ventricle of each side are in communication through the atrio- 

 ventricular foramen, the two sides are in communication only by the fora- 

 men ovale which is but a temporary opening. 



After the opening of the sinus venosus is shifted to the right atrium, the left 

 atrium for a short period has no vessels opening into it. As soon, however, as 

 the pulmonary veins develop, they form a permanent union with the left atrium 

 (Fig. 202). At first two veins arise from each lung, which unite to form a single 

 vessel on each side; the two single vessels then unite to form a common trunk 

 which opens into the left atrium on the cephalic side. As development pro- 

 ceeds, the wall of the single trunk is gradually absorbed in the wall of the atrium, 

 until the single vessel from each side opens separately. Absorption continu- 

 ing, all four veins, two from each lung finally open separately. This is the con- 

 dition usually found in the adult. A partial failure in the absorption may leave 

 one, two, or three vessels opening into the atrium. Such variations are not 

 infrequently met with in the pulmonary veins. 



The Valves. If all the passageways between the' different chambers of 

 the heart and the large vascular trunks were to remain free and clear, there 

 would be nothing to prevent the blood from flowing contrary to its proper course. 

 Consequently five sets of valves develop in relation to these orifices, and are so 

 arranged that they direct the blood in a certain definite direction. These ap- 

 pear (a) at the openings of the large venous trunks into the right atrium, (b) at 

 the opening between the right atrium and j^ght ventricle, (c) at the opening 

 between the left atrium and left ventricle, (d) at the opening between right 

 ventricle and pulmonary artery and (e) at the opening between the left ven- 

 tricle and aorta. No valves develop at the openings of the pulmonary veins 

 into the left atrium. 



(a) The sinus venosus (which is formed by the union of the large body veins) 

 opens into the right atrium on its cranial side, as has already been mentioned 

 (p. 228). By a process of absorption, similar to that in the case of the pul- 

 monary veins, the wall of the sinus is taken up into the wall of the atrium. The 

 result is that the vena cava superior, vena cava inferior, and sinus coronarius 

 (a remnant of the left duct of Cuvier) open separately into the atrium. As the 

 sinus is absorbed, its wall forms two ridges on the inner surface of the atrium, 

 one situated at the right of the opening and one at the left (Figs. 201 and 202). 

 These two ridges valvula venosce are united at their cranial ends with the 

 septum spurium (Fig. 200), a ridge projecting from the cephalic wall of the 

 atrium. The septum spurium probably has a tendency to draw the two valves 



