IV.] THE AORTIC ARCHES. 81 



meet at points opposite the head and opposite the tail, 

 thus all but encircling the vascular area between them. 

 At the point opposite the head the end of each semi- 

 circle is connected with vessels (Fig. 23), which run straight 

 in towards the heart along the fold of the splanchnopleure, 

 and join the right and left omphalo-mesaraic veins. At 

 the point opposite the tail there is at this stage no such 

 definite connection. At the two sides, midway between 

 their head and tail ends, the two semicircles are espe- 

 cially connected with the omphalo-mesaraic arteries. 



The circulation of the blood then during the latter half 

 of the second day may be described as follows. The blood 

 brought by the omphalo-mesaraic veins falls into the twisted 

 cavity of the heart, and is driven thence through the bulbus 

 arteriosus and aortic arches into the aortic trunk. From the 

 aorta, by far the greater part of the blood Hows into the 

 omphalo-mesaraic arteries, only a small remnant passing on 

 into the caudal terminations. From the capillary net-work 

 of the vascular and pellucid area into which the ompljalo- 

 mesaraic arteries discharge their contents, part of the blood 

 is gathered up at once into the lateral or direct trunks 

 of the omphalo-mesaraic veins. Part however goes into 

 the_ middle region of each lateral half of the sinus termi- 

 nalis, and there divides on each side into two streams. One 

 stream, and that the larger one, flows in a forward direction 

 until it reaches the point opposite the head, thence it 

 returns by the veins spoken of above, straight to the 

 omphalo-mesaraic trunks. The other stream flows backward, 

 and becomes lost at the point opposite to the tail. This is 

 the condition of things during the second day ; it becomes 

 considerably changed on the succeeding day. 



At the time that the heart first begins to beat the 

 capillary system of the vascular and pellucid areas is not yet 

 completed ; and the fluid which is at first driven by the heart 

 contains, according to most observers, very few corpuscles. 



20. At the close of the second day the single pair of 

 aortic arches into which the bulbus arteriosus divides is 

 found to be accompanied by a second pair, formed in the 

 same way as the first, and occupymg a position a little 

 behind it. Sometimes even a third pair is added. Of these 

 aortic arches we shall have to speak more fully later on. 

 E. 6 



