884 



DEVELOPMENT OF THE AORTIC ARCHES. 



arterv and are disposed towards each other like the tubes in a double-barrelled ^gun. The 

 .septum ctows downwards until it meets the ventricular septum (5), so that the right ventricle 

 comes to be connected with the pulmonary artery, and the left with the aorta. The division of 

 the truncus arteriosus, however, takes place only in the first part of its course. The division 

 does not take place above, so that the pulmonary artery and aorta unite in one common trunk 

 alove. Tliis communication between the pulmonary artery and the aorta is the ductus 



arteriosus Botalli (7, B). a w j j- * n ua. 



In the auricle a septum grows from the front and behind, ending internally with a concave 

 niarL'iii The vena cava superior (6, Cs) terminates to the right of this fold, so that its blood 

 will tend to go towards the right ventricle, in the direction of the arrow in 6, x. The cava 

 inferior, on the other hand (6, Ci), opens directly opposite the fold. On the left of its orifice 

 the valve of the foramen ovale is formed by a fold growing towards the auricular fold, so that 

 the blood-current from the inferior vena cava goes only to the left, in the direction of the 

 arrow, y ; on the right of the orifice of the cava, and opposite the fold, is the Eustachian valve, 

 which, in conjunction with the tubercle of Lower (tL), directs the stream from the inferior 

 vena cava to the left into the left auricle, through the pervious foramen ovale. Compare the 

 fcetal circulation (p. 876). After birth, the valve of the foramen ovale closes that aperture, 

 while the ductus arteriosus also becomes impervious, so that the blood of the pulmonary artery 

 is forced to go through the pulmonary branches proceeding to the expauding lungs. Some- 

 times the foramen ovale remains pervious, giving rise to serious symptoms after a time, and 

 constituting morbus ceruleus. 



Arteries. With the formation of the branchial arches and clefts, the number of aortic arches 

 on each side becomes increased to 5 (fig. 676), which run above and below each branchial cleft, 



1. 



Fig. 676. 

 The aortic arches. 1. The first position of the 1, 2, and 3 arches. 2. 5 aortic arches ; to, 

 common aortic trunk ; ad, descending aorta. 3. Disappearance of the upper two arches on 

 each side S, subclavian artery ; v, vertebral artery ; ax, axillary artery. 4. Transition 

 to the final stage P, pulmonary artery ; A, aorta ; dB, ductus arteriosus (Botalli) ; S, 

 right subclavian, united with the right common carotid, which divides into the internal 

 (Ci) and external carotid (Ce) ; ax, axillary ; v, vertebral artery. 



in a branchial arch, and then all reunite behind in a common descending trunk (2, ad) (Rathkc). 

 These blood-vessels remain only in animals that breathe by gills. In man, the upper two 

 arches disappear completely (3). When the truncus arteriosus divides into the pulmonary 

 artery and the aorta (4, P, A), the lowest arch on the left side, with its origin, forms the 

 pulmonary artery (4), and it springs from the right side of the heart. Of these the left, lowest 

 arch forms the ductus arteriosus (dB), and from the commencement of the latter proceed the 

 pulmonary branches of the pulmonary artery. Of the remaining arches which are united with the 

 aorta, the left middle one (i.e., the fourth left) forms the permanent aortic arch into which the 

 ductus arteriosus opens, while the right one (fourth) forms the subclavian artery; the third 

 arch forms on each side the origin of the carotids (Ci, Ce). The arteries of the first and second 

 circulations have been referred to already (p. 870). When the umbilical vesicle, with its 

 primary circulation, diminishes, only one omphalo-mescnteric artery is present, which gives 

 a branch to the intestine. At a later period, the omphalo-mesenteric arteries atrophy, while 

 the artery to the intestine the superior mesenteric becomes the largest of all, it being 

 originally derived from one of the omphalo-mesenteric arteries. 



Veins of the Body. The veins first formed in the body of the embryo itself are the two 

 cardinal veins ; on each side an anterior (tig. 677, I, cs), and a posterior (ci), which proceed 

 towards the heart and unite on each side to form a large trunk, the duct of Cuvier (DC), which 

 passes into the venous part of the heart. The anterior cardinal veins give off the subclavian 

 veins (bb) and the common jugular veins, which divide into the external (Ie) and internal 

 (Ji) jugular veins. In addition, there is a transverse anastomosing branch passing obliquely 

 from the left (where it divides) to the right, which joins their trunk lower down. In the final 



