996 



A MANUAL OF ANATOMY 



aortic bulb is divided into the ascending aorta and trunk of the pulmonary 

 artery, each lateral cushion is divided into two equal segments — anterior 

 and posterior — by the aortic septum as it descends. The aortic and pul- 

 monary orifices are thus provided each with three valves. The pulmonary 

 valves are disposed as follows: One is anterior in position, and it represents 

 the original anterior cushion of the lower end of the undivided aortic bulb; 

 whilst two are postero-lateral, right and left, and they represent the anterior 

 segments of the lateral cushions of the lower end of the undivided aortic 

 bulb. The aortic valves are disposed as follows: Two are antero-lateral, right 

 and left, and they represent the posterior segments of the lateral cushions 

 of the lower end of the undivided aortic bulb ; whilst one is posterior in position, 

 and it represents the posterior cushion of the lower end of the undivided 

 aortic bulb. 



At this stage the root of the pulmonary artery is in front of the root of 

 the aorta, and the segments of the pulmonary and aortic valves occupy the 



PULMONARY 

 ARTERY 



PULMONARY 

 ARTERY 



Endocardial Cushions 

 of Primitive Aortic 

 Bulb 



Aortic Bulb undergoing 



Division : Segmentation 



of Lateral Cushions 



R.P.L. = Right Postero-Lateral 

 R,A.L. = Right Antero- Lateral 



Fig. 411. — Development of Aortic and Pulmonary Valves. 



positions which have just been assigned to them. Towards the end of intra- 

 uterine life, however, the heart undergoes a rotation from right to left to 

 such an extent as to place the root of the pulmonary artery in front, and to 

 the left, of the root of the aorta. The aorta, as well as the pulmonary artery, 

 is involved in this rotation, and the segments of the pulmonary and aortic 

 valves are now permanently altered in position. The relations of the various 

 segments in the adult are as follows: The pulmonary valves are disposed as 

 two antero-lateral, right and left, and one posterior; whilst the aortic valves are 

 one anterior and two postero-lateral, right and left. 



The rotation, just referred to, also explains why the left ventricle of the 

 adult heart is chiefly postero-inferior in position, comparatively Uttle of it 

 appearing on the antero- superior surface of the adult heart. 



Peculiarities of the Foetal Circulation. — The structures peculiar 

 to the foetus in connection with the circulation of the blood are as 

 follows : 



I. Foramen ovale, 

 3. Eustachian valve. 

 3. Ductus arteriosus. 



4. Hypogastric and umbilical arteries. 



5. Umbilical vein. 



6. Ductus venosus. 



The foramen ovale is an oval opening in the interauricular septum. 

 Up to the fourth month of intra-uterine life it is quite free, but after 

 that joeriod the valve of the foramen ovale gradually extends upwards 

 on the left side of, and beyond, the annulus ovalis, and acts as a 

 valve during the latter months of foetal life. The blood can then 



