DEVELOPMENT OF THE VASCULAR SYSTEM 



99 I 



both ventricles, lying between an anterior and a posterior lip of projecting endothe- 

 lium, from, which the auriculo ventricular valves are formed. The ventricular sep- 

 tum grows upward toward the auricular canal and fuses at this point with two endo- 

 thelial proliferations (endothelial cushions), which in the lumen of the auricular 

 canal traverse the mouth of the canal from before and behind. In the eighth 

 week the ventricular septum is fully developed. A free communication thus 

 exists between the large undivided auricle and the corresponding ventricle (5) 

 by way of a right and a left auriculo-ventricular orifice. There then grow into 

 the large truncus arteriosus (4, p) two wing-like septa (4, p a), which finally 

 meet in union and divide the tube into two tubes (5 , a p). The latter lie parallel 

 to one another like the barrels of a double-barreled gun (aorta and pulmonary 

 artery). The septum between the two assumes a downward direction in such a way 

 as to meet the interventricular septum (5). In this way the right ventricle 

 communicates with the pulmonary artery and the left with the aorta. The divi- 

 sion of the truncus arteriosus, however, takes place only in its first part. Further 

 on, it is not complete, that is the pulmonary artery and the aorta unite above 

 to form a common trunk the ductus arteriosus Botalli (7, B). In the auricle 

 thera gro-vs from before and bahind a portion of a septum that ends within the 

 cavity in a concave border. The superior cava (6, Cs) enters to the right of this 

 fold, so that its blood would have a tendency to pass into the right ventricle (in 

 the direction of the arrow 6, x). The inferior cava (6, Ci) , on the contrary, opens 

 directly opposite the margin of the fold. There is thus formed to the left of its 



t. 



FIG. 385. Development from the Aortic Arches: i, The rudimentary ist, 2d, and 3d aortic arches. 2, Five 

 aortic arches: fa, common aortic trunk; a d, descending aorta. 3, Atrophy of the two uppermost arches 

 on each side; S, subclavian artery; v, vertebral artery; ax, axillary artery. 4, Transition to the definitive 

 stage of formation; P, pulmonary artery; A, aorta; dB, ductus Botalli; S, right subclavian artery, joined 

 with the right common carotid, which divides into the internal (Ci) and the external (Ce) carotid; ax-, axillary 

 artery; v, vertebral artery. (Diagrammatic.) 



entrance, opposite the auricular fold, the valve of the oval foramen, which permits 

 the blood to flow only to the left in the direction of the arrow y. To the right 

 of the mouth of the cava, opposite the fold, there is formed the Eustachian valve, 

 w.iich together with the tubercle of Lower (tL) guides the stream of the inferior 

 cava to the left into the left auricle. After birth the opening is closed by the 

 valve of the oval foramen. In addition, the ductus Botalli is obliterated (by 

 increase of the pressure in the aorta, which closes the lumen of the mouth), so 

 that the blood of the pulmonary artery is now forced to pass through the distend- 

 ing pulmonary branches. 



The persistence of the oval foramen is a developmental defect that causes 

 severe circulatory disturbances. Kergeradek discovered the fetal heart-sounds. 

 Many anomalous peculiarities in the development of the heart, which have been 

 studied especially by Born and Rose, cannot be described here. 



Arteries. With the development of the visceral arches and clefts, the num- 

 ber of aortic arches on each side becomes increased from one up to five (Fig. 385), 

 and these pass above and below each visceral cleft, but subsequently unite to form 

 a common trunk (2, ad). The vessels persist only in gill-breathers, Fig. 74. In 

 man, the two uppermost aortic arches on each side (3) atrophy first. In the division 

 of the truncus arteriosus into the pulmonary artery and the aorta (4, PA) the lower- 

 most arch on each side, together with its origin, becomes the pulmonary artery 

 (4) and therefore arises from the right heart. Of these, the left lowermost arch 

 forms the ductus -Botalli (dB), and at its origin the pulmonary branches of the 

 pulmonary artery arise. Of the arches joined to the aorta, the left middle one 



