470 MALL. [Vol. V. 



third, fourth, and the tissue aboralwards from the fourth branchial 

 arches. They are the third, fourth, and fifth aortic arches. 

 Their general direction and shape is shown on Plate XXX. 

 The two sets of arches unite to form the two aortae, which again 

 unite, between the sixth and seventh cervical nerves, to form 

 a single aorta. In His's embryo B the division is about at the 

 same point ; l while in embryo A it is at the fourth dorsal. 2 

 As the aorta passes backward, it gradually becomes larger and 

 larger, so that its diameter in the lumbar region is several times 

 that in the cervical. 3 Here it very abruptly breaks up into 

 two large branches which pass into the cord as the umbilical 

 arteries. 



Although there are many blood corpuscles scattered through- 

 out the tissue of the embryo, I can make out definitely only a 

 few arteries. The artery arising from the third aortic arch, 

 and passing along the dorsal side of the branchial cavity up to 

 the eye, is undoubtedly the internal carotid. Slightly beyond 

 the eye it breaks up into numerous small branches, the most 

 prominent passing towards the mid-brain, and undoubtedly 

 represents the posterior cerebral. In His's diagram 4 this same 

 twig passes between the inter- and mid-brains, and this throws it 

 in front of the third nerve. In the neighborhood of the eye no 

 branches could be found which arise from the carotid, but a 

 large branch passes through the retina. This indicates that 

 the ophthalmic is present, but cannot be followed in the 

 sections. 



From the fifth aortic arch, on either side, there is a branch 

 which passes to the lung, and breaks up into a network of 

 capillaries about the pulmonary buds. This is the pulmonary 

 artery. 



On the dorsal side of the aorta there are, on either side, 

 twenty-one segmental arteries, the first being in front of the 

 first cervical nerve, and the last behind the twelfth dorsal. 

 The second segmental communicates on either side with a large 

 branch — the vertebral. This branch extends as far forward as 



1 Atlas, PI. I., Figs, i and 3. 



2 Replace Figs. 70 and 72, PI. V., Atlas, into Fig. 4, PI. I. 



3 See, also, His's embryo R, Atlas, PI. XII.; embryo B, Pis. II., III.; and 

 embryo A, PI. V. 



4 An. mensch. Em., III., Fig. 122, S. 188. 



