THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



209 



to function and the placental blood is cut off, so that the right atrium receives 

 venous blood only and the left arterial blood only. If the foramen ovale were 

 to persist it would allow a mingling of venous and arterial blood. Con- 

 sequently the foramen ovale closes soon after birth and the two currents of 

 blood are completely separated. At the same time the ductus arteriosus 

 atrophies and becomes the ligamentum arteriosum. Consequently there is 

 no direct communication between the pulmonary artery and aorta. 



Certain features of development have an important bearing on the theories regarding 

 the physiology of the heart, particularly on the theory that the heart is an automatic 

 organ. Whether the theory that the heart beats automatically, i.e., independently of 

 stimuli from the nervous system, is true or not, it is a fact that in the embryo it begins to 

 beat before any nerve cells appear in it and before any nerve fibers are connected with it. 

 At least no technic has yet been devised by which it is possible to demonstrate nerve cells 

 in, or fibers connected with it, at the time when it begins to perform its characteristic 

 function. And, furthermore, at the time when the heart begins to beat, no heart muscle 

 cells are developed. This last fact seems to indicate an inherent contractility in the 

 mesothelial cells which form the anlage of the myocardium. 



The Arteries. The simplest condition of the arterial system, following 

 the establishment of the vitelline and allantoic circulation (p. 189 and p. 



Dors, aortic root 



Vent, aortic root 



Vent, aortic trunk 



Dors, aortic root 



(Esophagus 



Trachea 



Pulmonary artery 



FIG. 180. From reconstruction of aortic arches (i, 2, 3, 4, 6) of left side and pharynx 



ot a 5 mm. human embryo. Tandler. 



I-IV, Inner branchial grooves. 



191), is as follows: The single ventral aortic trunk is given off from the 

 cephalic end of the heart. This is a short vessel, soon dividing into the 

 two vejntral aortic roots which pass forward beneath the pharynx (Fig. 180). 

 Each ventral aortic root gives rise to branches which pass dorsally, one in 

 each branchial arch, as the aortic arches to unite in a common stem along 

 the dorsal wall of the pharynx. This common stem is the dorsal aortic 

 root (Fig. 1 80) which fuses with its fellow of the opposite side in the mid- 

 dorsal line to form the dorsal aorta. The single dorsal aorta, situated 

 ventral to the notochord, extends from the cervical region to the caudal 

 end of the embryo. Somewhat caudal to the middle of the embryo a branch 



