8 4 4 



EMBRYOLOGY 



nary artery and the ductus arteriosus. The fifth arch on the right 

 side disappears. These changes occur in man and in some mammals ; 

 but in fishes the five arches are permanent, giving off branches that 

 are distributed to the gills. In following the metamorphoses of the 

 arches in man, it is seen that the symmetrical vascular system that 

 exists in early embryonic life is developed gradually into an unsym- 

 metrical system, and that two arches the fourth and fifth soon 

 far exceed the others in size and importance. The destination of 

 the five arches is well illustrated diagrammatically in Fig. 245, modi- 

 fied from Rathke. The aortic bulb and the five arches are represented 



COVMON 

 CAROTI 



LEFT 

 SUBCLAVIAN 



DUCTUS 

 ARTLRIO6U8 



LMONARY 

 TRUNK. 



DESCENDING AORTA 



Fig. 245. Arterial arches in man and mammals (modified from Rathke). 



in outline and the permanent vessels in colors. The vessels belonging 

 to the aortic system are in red and the pulmonary vessels are in blue. 



At the same time that the arteries going to the head and upper 

 extremities are undergoing development, the vessels of the trunk and 

 lower extremities are branching from the descending aorta and its exten- 

 sions. These processes, however, are not complex and do not call for 

 extended description in this work. 



Two venous trunks make their appearance by the sides of the spinal 

 column, called the cardinal veins. These run parallel with the superior 

 vertebral arteries, or the two aortae, emptying finally into the auricular 

 portion of the heart by two canals, called the ducts of Cuvier. These 

 veins change their relations and connections as the first circulation is 

 replaced by the second. The omphalo-mesenteric vein opens between 



