THE DEVELOPMENT OF THE VASCULAR SYSTEM. 205 



development proceeds, 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 continuing, all four veins, two from each lung finally open 

 separately. This is the condition 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 appear (a) at the openings of the large venous trunks into the right 

 atrium, (b) at the opening between the right atrium and right 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 open- 

 ing between the left ventricle 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. 201). By a process of absorption, similar to that in the case 

 of the pulmonary 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. 172 and 173). These two ridges valvulce venosce are 

 united at their cranial ends with the septum spurium (Fig. 171), a ridge 

 projecting from the cephalic wall of the atrium. The septum spurium 

 probably has a tendency to draw the two valves together and prevent the 

 blood from flowing back into the veins. The left valve and the septum 

 spurium later atrophy to a certain extent and probably unite with the septum 

 atriorum to form part of the limbus fossce ovalis (Vieussenii) . The right 

 valve is the larger and in addition to its assistance in preventing a backward 

 flow of blood into the veins, it also serves to direct the flow toward the 

 foramen o\;ale. As the veins come to open separately, the cephalic part 

 of the right valve disappears; the greater part of the remainder becomes 

 the valvula Deuce cavce inferioris (Eustachii) and during fcetal life directs the 

 blood toward the foramen ovale. In the adult it becomes a structure of 

 variable size. A small part of the remainder of the right valve forms the val- 

 vula sinus coronarii (Thebesii) which guards the opening of the coronary sinus. 



