VIII.] THE SUBCLAVIAN ARTERIES. . 217 



describing. This vessel 'passes up in the neural arch of the vertebrae, and is 

 usually spoken of as the vertebral artery. 



Eathke calls the vertebral arteries the 'arteriEe collaterales colli,' and if his 

 view of their development is correct they can scarcely be considered homologous 

 with the vertebral arteries usually so called in mammals. Soon after the ana- 

 stomosis between the third and fourth arches disappears, the common trunk of 

 the carotids on each side becomes much lengthened, and it is from near the base 

 of the lengthened common carotid that the 'vertebral arteiy' (as we shaU call 

 Eathke's ' arteria coUateralis colli ') takes its origin. 



The fourth arch of the right side now becomes the most 

 important of all the arches ; and nearly the whole of the 

 blood supplj'^ing the hinder parts of the body passes through 

 it. It is this arch which remains as the permanent aortic 

 arch of the adult; and it is important to notice that the 

 arch which forms the great dorsal aorta in birds is the fourth 

 on the right side, and not as in mammals the fourth on the 

 left side. 



Prom the observations of Eathke we know, with tolerable certainty, the 

 manner in which the carotids and the so-called vertebral arteries of birds are 

 developed. There is however still some doubt as to the origin of the subclavian 

 arteries, although Von Baer and Rathke have both investigated the point. 



Von Baer believed that the artery which forms the continuation of the third 

 arch, which on Rathke's authority we have called the 'internal carotid,' 

 became the 'vertebral artery;' and he believed that the subclavian was given 

 off as a branch from it. As Rathke points out, this does not agree with the 

 anatomy of the parts, since in birds the subclavian forms the continuation of 

 the innominate artery, after a common branch for the vertebral and carotids 

 has been given otf trom it. If it had not been for Rathke's satisfactory 

 observations on the development of the carotids and vertebrals, this would not 

 be a fatal objection to Von Baer's view; since we might easily suppose that 

 although the subclavian was originally a branch of the vertebral, yet by sub- 

 sequent changes, the point at which the subclavian left the vertebral was 

 carried further and further back, till finally the subclavian became a branch 

 of the common trunk of the vertebral and carotids, or in other words the 

 subclavian formed the continuation of the innominate artery, after the com- 

 mon branch which divides into the vertebral and carotids had been given off 

 from it. 



Rathke's view of the origin of the subclavian is founded on the analogy of 

 other vertebrates, rather than on his own observations on the chick. He states 

 that although he attempted to do so, he was unable satisfactorily to observe the 

 origin of these vessels ia the chick. The following is the view which he adopts, 

 and which we have followed in our diagram (Fig. 66). 



The right subclavian (R.sc.) arises, he believes, either from the connecting 

 branch (anastomosis) between the fom-th and fifth arches (of that side), or from 

 the branch connecting the fifth arch with the dorsal aorta ; probably from the 

 former. This would make its development very nearly similar to the develop- 

 ment of the corresponding subclavian {i.e. the left) in mammals. We shall 

 mention directly, in speaking of the final changes which the arterial system 

 undergoes, how it is that the right subclavian finally comes to form the continu- 

 ation of the right innominate artery. 



The left subclavian {L. sc.) forms the continuation of the fourth arch 



