DEVELOPMENT OF THE CIRCULATORY APPARATUS. 837 



Corresponding to the four visceral arches, which have been described in 

 connection with the development of the face, are four vascular arches. One 

 of these disappears, and the remaining three undergo certain changes, by 

 which they are converted into the vessels going to the head and the superior 

 extremities. The anterior arches on the two sides are converted into the 

 carotids and subclavians ; the second, on the left side, 

 is converted into the permanent aorta, and the right is 

 obliterated ; the third, on either side, is converted into 

 the right and left pulmonary arteries. 



The changes of the branchial arches are illustrated 

 in the diagrammatic Fig. 314. In this figure the three 

 branchial arches that remain and participate in the de- 

 velopment of the upper portion of the vascular system 

 are 1, 2, 3, on either side. The two anterior (3, 3) be- 

 come the carotids (c, c) and the subclavians (s, s). The 

 second (2, 2) is obliterated on the right side, and be- 

 comes the arch of the aorta on the left side. The third 

 (1, 1), counting from above downward, is converted into 

 the pulmonary arteries of the two sides. Upon the left 

 side there is a large, anastomosing vessel (c), between 

 the pulmonary artery of that side and the arch of the 

 aorta, which is the ductus arteriosus. The anastomos- 

 ing vessel (cd), between the right pulmonary artery and 

 the aorta, is obliterated. 



The mode of development of the veins is very sim- 

 ple. Two venous trunks make their appearance by the 

 sides of the spinal column, which are called the cardi- 

 nal veins, and run parallel with the superior vertebral 

 arteries, or the two aortse, emptying finally into the au- 

 ricular portion of the heart, by two canals, which are 

 called the canals of Cuvier. These veins change their 

 relations and connections as the first circulation is re- 

 placed by the second. The omphalo-mesentefic vein opens into the heart, 

 between the two canals of Cuvier. As development advances, the liver is 

 formed in the course of this vessel, a short distance below the heart, and the 

 vein ramifies in its substance ; so that the blood of the omphalo-mesenteric 

 vein passes through the liver before it goes to the heart. The omphalo- 

 mesenteric vein is obliterated as the umbilical vein makes its appearance. 

 The blood from the umbilical vein is at first emptied directly into the heart ; 

 but this vessel soon establishes the same relations with the liver as the om- 

 phalo-mesenteric vein, and its blood passes through the liver before it reaches 

 the central organ of the circulation. As the omphalo-mesenteric vein atro- 

 phies, the mesenteric vein, bringing the blood from the intestinal canal, is 

 developed, and this penetrates the liver, becoming finally the portal vein. 



As the lower extremities are developed, the inferior vena cava makes its 

 appearance, between the two inferior cardinal veins. This vessel receives an 



FIG. 314. Transforma- 

 tion of the system of 

 aortic arches into per- 

 manent arterial trunks, 

 in the mammalia (Von 

 Baer). 



B, aortic bulb : 1, 2, 3, 4, 

 5, on either side, the five 

 pairs of aortic arches ; 

 5, 5, the earliest in their 

 appearance ; 1, 1, the 

 most recent ; c, c, the 

 two carotids, still unit- 

 ed, which are separated 

 at a later period : .s, s, 

 the two subclavians, the 

 right arising from the 

 arteria innominata ; a, 

 a, the aorta ; p, p, the 

 pulmonary arteries ; ca, 

 the ductus arteriosus ; 

 cd, the left artieral ca- 

 nal, which is finally ob- 

 literated. 



