7o8 



HUMAN ANATOMY. 



Aorta 



Pharynx 



on the contrary, persists and differentiates into the larger Eustachian valve, that 

 guards the lower margin of the inferior vena cava and directs its blood-stream towards 

 the foramen ovale, and the smaller Thebesian valve, that protects the orifice of the 

 coronary sinus. 



As above noted, the separation of the two auricles is incomplete on account of 

 the existence of the foramen ovale within the interauricular partition. From the roof 

 and anterior wall of the right auricle an additional and relatively thick crescentic ridge, 

 the septum secundum, arches around the foramen ovale of which it forms the anterior 

 or \entral boundary. It lies close to and parallel with the auricular septum and 

 fuses below with the lower part of the left segment of the venous valve to form the 

 limbus Vieusseyiii that later limits the fossa ovalis, marking the former position of the 

 foramen ovale. The latter, therefore, is included between two partially overlapping 

 crescentic margins, that contributed by the septum auriculum lying behind and to the 

 left, and that by the septum secundum in front and to the right, a narrow sagittal 



cleft intervening so that the surfaces of 

 .Fig. 671. the lunate borders are not in contact. 



Since the division of the heart into 

 a right and left side is inseparably con- 

 nected with the development of the 

 lungs and the consequent necessity for 

 a distinct pulmonary circulation, provis- 

 ion for the return of the blood from the 

 lungs to the heart is made by the early 

 formation of the pulmonary veins. 

 These arise in pairs, one pair for each 

 lung; close to the heart each pair unites 

 into a single right or left stem that, in 

 turn, joins with its fellow of the oppo- 

 site side to form one short common 

 trunk. For a time none of these ves- 

 sels communicate with the heart, but 

 later the common single pulmonary vein 

 opens into the left auricle close to the 

 septum. With the subsequent growth 

 and expansion of the auricles an appro- 

 priation occurs on the left side similar to 

 that affecting the sinus venosus on the 

 right, in consequence of which the short 

 common pulmonary vein is first drawn 

 into the heart, to be followed next by 

 the two secondary and, finally, by the 

 four primary pulmonary veins, all of 

 which then open by separate orifices 

 into the enlarging left auricle. The fre- 

 quent variations in the number of the pulmonary veins and in their relations to the 

 heart are usually to be referred to arrest or modification of this foetal appropriation. 



The differentiation of a right and left auriciilo-ventricular valve proceeds from the 

 subdivision of the auricular canal by the septum intermedium. The latter is formed 

 by the approximation and union of the median cushion-like projections upon the 

 ventral and dorsal walls of the common auriculo-ventricular opening. The unob- 

 literated lateral portions of the latter are triangular in outline and guarded by pro- 

 liferations of the endocardium. Those of the lower margins of the valves elongate 

 and project into the ventricles on the right side, giving rise to two leaflets, and on the 

 left to a single flap. An additional prolongation from the partition contributes a 

 septal leaflet on each side. In this manner the complement of flaps for the tricuspic^ 

 and bicuspid (mitral) valves is early provided. The close relation between the leaf- 

 lets and the attached restraining bands, the chordae tendineae, results from the 

 secondary union of the immature flaps with the trabeculse of the spongy myocardium 

 of the young heart. The loose muscular walls undergo partial consolidation, so that 



Auricles 



Auricular 

 canal 



Endocar- 

 dium 



Ventricle 





Myocar- 

 dium 



Truncus arteriosus 



Transverse section of somewhat older em- 

 bryo, showing differentiation into auricles, 

 ventricle and truncus arteriosus. X 75. 



