VIII.] THE CAKOTIDS. 213 



flowing through them is entirely shut oflf from the left auricle, 

 into which passes the blood from the two pulmonary veins 

 (Fig. 64, Z.F.). 



Such is the history of the veins in the chick. As will be 

 seen in the second part of this work, the course of events in 

 the mammal, though in the main similar, differs in some 

 unimportant respects. 



10. It remains for us to speak of the changes which have 

 in the meantime been taking place in the arterial system. The 

 condition of things which exists on the fifth or sixth day is 

 shewn in the diagram (Fig. 65). 



We have already seen (Chap. VI. p. 168) that of the three 

 aortic arches which make their appearance on the third day, 

 the first two disappear : the first on the fourth, the second on 

 the fifth day ; but that their disappearance is accompanied 

 by the formation behind them of two new aortic arches, the 

 fourth and the fifth. Thus there are generally three, never 

 more than three, pairs of aortic arches present and functional 

 at one time. 



According however to Von Baer this is not strictly true. He statei; that 

 there are four arches present both on the fourth and fifth days. In the case 

 of the fourth day, a slight remnant of the first pair of arches still persists when 

 the fourth pair is already formed ; and on the fifth day the third pair has not 

 entirely disappeared when the fifth pair is formed. In both of these cases 

 however the first pair of arches of the four is only present for a very short time, 

 and then is so diminished in size as to be of no importance. 



The first pair of arches, before it entirely disappears, sends 

 off on each side two branches towards the head. Of these, 

 one forms the direct continuation of the bulbus arteriosus in 

 a straight line from the point where the first aortic arch 

 leaves it. Primarily distributed to the tongue and inferior 

 maxillary region, it becomes the external carotid (Fig. 65, 

 E.CA). 



The other, starting from the point where the aortic arch 

 of each side joins its fellow above the alimentary canal to form 

 the dorsal aorta, is primarily distributed to the brain, and 

 becomes the internal carotid (Fig. 65, 1.CA). 



When the first arch disappears, the external carotid 

 arteries still reniain as the anterior continuations of the 

 bulbus arteriosus. And since the dorsal trunks uniting the 

 distal ends of the first and second arches do not become 

 obliterated at the time when the first pair of arches dis^ 



