78 F. W. THYNG 



real continuation of the ventral ramus of the seventh cervical 

 intersegmental. The continuation of the subclavian beyond the 

 point of origin of the internal mammary appears to represent a 

 lateral branch of the ventral ramus of the seventh intersegmental. 

 By the anomaly found on the right side of this embryo, the deep 

 cervical and superior intercostal arteries come to have separate 

 origins from the subclavian as sometimes occur in the adult. 



Parietal arteries of the trunk. From the level of the second 

 pair of thoracic nerves, where the dorso-lateral aortae unite, the 

 single dorso-median aorta (Ao.d.m.) continues throughout the 

 trunk to end as the middle sacral (A.s.m.). The dorsal inter- 

 segmental arteries arise in regular pairs from the dorso-median 

 aorta. Those of the right side are shown in plate 1, the corre- 

 sponding vessels on the left having been omitted. 



The dorsal intersegmental arteries curve laterally and dorsally, 

 passing lateral to the sympathetic, and either pass dorsally to 

 become muscular branches or continue medially as spinal rami 

 into the vertebral canal. In the latter situation the spinal 

 rami pass laterally to the spinal cord to form the posterior spinal 

 arteries or join to form a bilateral ventral longitudinal anasto- 

 mosis which extends from the vertebral artery (directly caudad 

 of the myelencephalon) along the spinal cord into the tail process. 

 Some of the spinal rami bifurcate contributing thereby a branch 

 to each anterior and posterior spinal artery of that side. 



That the embryonic right anterior spinal anastomosis eventually 

 joins its fellow of the opposite side to form the single anterior 

 spinal artery of the adult was first indicated by His ('86). Re- 

 cently the matter has been treated in greater detail by Sterzi 

 ('04), and Evans ('09 and '12). The latter observers conclude 

 that fusion between the two stems does not occur, but that the 

 single adult artery represents the selected channel persisting 

 from the alternative paths offered by the two longitudinal vessels 

 and the plexiform connections between them. 



In the thorax and abdomen the ventral rami of the dorsal inter- 

 segmentals become the aortic intercostal and lumbar arteries, 

 respectively. Some of the f orm.er are represented as short stumps 

 in plate 1. The ventral rami of the first and second thoracic 



