THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



207 



sidered in connection with the development of the pericardium (Chap. XIV). 

 With the exception of the septum atriorum, the heart acquires during fcetal 

 life practically the form and structure characteristic of the adult (Fig. 

 178). So long as the individual continues to grow, the heart, generally 

 speaking, increases in size accordingly. This increase takes place by in- 

 tussusception in the endocardium and myocardium. At the time of birth 

 the two atria are in communication through the foramen ovale which is 



Atrial septum 



Right atrium 



Right atrio- 



ventricular 



(tricuspid) valves 



Right ventricle 



Ventricular 

 septum 



Pericardial cavity 



Dorsal aortic roots 



Amnioo 



Upper limb bud 



Left atrium 



Left atrio- 

 ventricular 

 (bicuspid) valves 



Left ventricle 



FIG. 177. Transverse section of pig embryo of 14 mm. Photograph. 



simply an orifice in the atrial septum (Fig. 179). Thus the blood which is 

 brought to the right atrium by the body veins is allowed to pass directly 

 into the left atrium, thence to the left ventricle, and thence is forced out to 

 the body again through the aorta. A certain amount of blood also passes 

 from the right atrium into the right ventricle and thence into the pulmonary 

 artery; but this blood does not enter the lungs but passes directly into the 

 aorta through the ductus arteriosus (Fig. 178). After birth the lungs begin 



