220 THE SIXTH DAY. [CHAP. 



reason is superfluous, and that the explanation is that the canalis Botalli of the 

 right side is merely the portion of the fifth arch between the origin of the 

 pulmonary artery and the junction of the fifth arch with that common trunk, 

 with which all the arches of that side are at one time or other connected, 

 and which remains as the continuation of the aortic arch into the dorsal aorta; 

 while on the left side it consists of the same portion of the arch as on the right 

 side, and also of the corresponding common trunk of the left side. 



When respiration commences tlie blood ceases to pass 

 through these canals, which either remain as mere ligaments 

 or else become absorbed altogether. By this means, the 

 foramen ovale becoming at the same time closed, a complete 

 double circulation is established. All the blood from the 

 right ventricle passes into the lungs, and all that from the 

 left ventricle into the body at large. 



Two other changes take place about the same time in 

 the aortic branches. That portion of the right fourth or 

 aortic arch which lies between the origin of the right sub- 

 clavian and the common carotid becomes shortened, and 

 is finally swallowed up in such a fashion that the right 

 subclavian (Fig. 67, R.sc.) comes off from the right common 

 carotid, a very short trunk being left between the union of 

 the two to serve as the right innominate artery. 



At the same time, corresponding to the increase in the 

 length of the neck, the common carotids are very greatly 

 lengthened. They lie close together in the neck, and in 

 many birds actually unite to form a common trunk. 



It will of course be understood that with the disap- 

 pearance of the allantois and the absorption of the yolk, 

 the umbilical and omphalo-mesaraic arteries also disappear. 



11. It may perhaps be of advantage to the reader if we 

 here briefly summarize the condition of the circulation at 

 its four most important epochs ; viz. on the third day, on 

 the fifth day, during the later days of incubation before 

 respiration by the lungs has commenced, and after the chick 

 has begun to breathe by the lungs. 



On the third day the circulation is of an exceedingly 

 simple character. 



The heart is to all intents and purposes a simple twisted 

 tube marked off by constrictions into a series of three con- 

 secutive chambers. The blood coming from the venous 

 radicles passes through the heart and then through the 

 three pair of arterial arches. 



