796 DEVELOPMENT OF THE VASCULAR SYSTEM. 



mously enlarged, giving off, as secondary branches, all the arterial 

 trunks going to the head and upper limbs, and is thus converted into 

 the permanent arch of the aorta. On the right side the corresponding 

 arch grows smaller, and ultimately disappears ; so that at last there is 

 only a single aortic arch, situated to the left of the median line, and 

 continuous below with the thoracic aorta. 



The fifth or last cervical arch becomes on each side the pulmonary 

 artery ; its external portion on the right side disappearing at a very 

 early period, but on the left remaining for a certain time, as the ductus 

 arteriosus, between the pulmonary artery and the aorta. 



Notwithstanding that the cervical arches are at first, as their name 

 implies, all situated in the region of the neck, their remains or perma- 

 nent representatives in the complete form of the arterial system, come 

 to be placed farther downward, and are evejn found in the cavity of the 

 chest. This is due to the varying rapidity of growth in different parts, 

 at the successive periods of embryonic development. The thorax at 

 first has no existence as a distinct portion of the trunk ; the heart 

 being placed immediately beneath the head, and afterward changing its 

 relative position as the development of the lungs goes forward and the 

 walls of the chest expand to cover them. The neck, with the esopha- 

 gus and trachea, also elongates in an upward direction, so that the vas- 

 cular organs at first placed in the cervical region afterward occupy a 

 position lower down. In fishes, where the cervical arches are perma- 

 nent and where no lungs are developed, there is no thoracic cavity, and 

 the heart remains situated at the most anterior portion of the trunk, just 

 behind the gills. 



Corresponding changes take place, during this time, in the lower part 

 of the body. Here the abdominal aorta runs undivided, upon the me- 

 dian line, quite to the end of the spinal column ; giving off on each side 

 lateral branches, which supply the intestine and the parietes of the body. 

 When the allantois begins to be developed, two of these lateral branches 

 accompany it, and become, consequently, the umbilical arteries. These 

 vessels increase so rapidly in size, that they soon appear as divisions 

 of the aortic trunk ; while the original continuation of the aorta, 

 running to the end of the spinal column, appears as a small branch 

 given off at the point of bifurcation. The lower limbs are supplied 

 by two small branches, given off from the umbilical arteries near their 

 origin. 



Up to this time, the pelvis and posterior extremities are but slightly 

 developed. Subsequently they grow more rapidly, in proportion to the 

 rest of the body, and the arteries which supply them enlarge in a corre- 

 sponding manner. That portion of the umbilical arteries, lying between 

 the bifurcation of the aorta and the origin of the branches going to the 

 lower extremities, becomes the common iliac arteries, which in their 

 turn afterward divide into the umbilical arteries proper, and the femorals. 

 Subsequently, in accordance with the continued growth of the pelvis 

 and lower extremities, the relative size of their bloodvessels is still 



