574 



EMBRYOLOGY. 



mentioned. The fourth arch (fig. 318 e) then remains in connection 

 with the trunk (d) which arises from the left ventricle and receives 

 blood exclusively from that source. The fifth arch (n), on the con- 

 trary, forms the continuation of that half (m) of the truncus arteriosus 

 which emerges from the right ventricle. Thus the division of the 

 blood into two separate currents initiated in the heart is also 

 continued into the nearest vessels, but for a short distance only, 

 since the fourth and fifth pairs of vascular arches (fig. 317) still 

 mpty their blood together into the aorta cominunis (ad), with the 

 exception of a certain portion which runs 

 through their accessory branches, in part to 

 the head (c.c') and upper limbs, in part to 

 the still diminutive lungs. Gradually, how- 

 ever, the process of separation thus introduced 

 is continued still farther into the region of 

 the peripheral vessels and finally leads to the 

 establishment of tJie entirely distinct major 

 and minor circulations. The final condition is 

 attained by the degeneration of certain portions 

 of the vessels and the enlargement of others. 



A preponderance of the vascular arches of 

 the left side over 'those of the right is soon 

 recognisable (fig. 318). The former con- 

 tinually increase in size, while those of the 

 right side become less and less apparent and 

 finally in places disappear altogether. They 

 are retained only in so far as they conduct 

 the blood to the lateral branches which, 

 arising from them, go to the head, the upper 

 limbs, and the lungs. Consequently of the 

 right aortic arch there remains only the 

 tract which gives rise to the right carotis communis (c) and 

 the right subclavia (i + l). We designate its initial part as the 

 arteria anoiiyma brachiocephalica. With this the permanent con- 

 dition is now established. The remnant of the right fourth vascular 

 arch appears as a side branch only of the aorta (e), which forms an 

 arch on the left side of the body, and here gives rise to the carotis 

 communis sinistra (c) and the subclavia sin. (h) as additional lateral 

 branches. 



The right half of the fifth [sixth] pair of vascular arches likewise 

 undergoes degeneration, except for the portion that conveys blood 



Fig. 318. Diagrammatic re- 

 presentation of the meta- 

 morphosis of the blood- 

 vessels of the visceral 

 arches in a Mammal, 

 after RATHKE. 



a, Carotis interna ; 6, carotis 

 externa ; c, carotis com- 

 munis ; d, body or sys- 

 temic aorta ; e, fourth 

 arch of the left side; 

 /, dorsal aorta ; g, left, 

 k, right vertebral artery ; 

 h, left subclavian artery ; 

 i, right subclavian (fourth 

 arch of the right side) ; 

 I, continuation of the 

 right subclavian ; m, pul- 

 monary artery ; n, its 

 ductus BotalH. 



