PARKER AND TOZIER : POSTCARDINAL VEINS IN SWINE. 



141 



which the accessory veins were developing, no trace of intercostals or 

 intercostal anastomoses could be discovered, but the accessory veins 

 grew at their postei'ior ends by a process that seemed like the formation 

 of an opening through the tissues independent of any pre-existing blood- 

 cavities. We therefore believe that Rathke was in error in ascribing to 

 the non-cardinal parts of the azygos and hemiazygos veins in swine 

 an origin from intercostal anastomoses. 



In justice to Rathke, however, it should be mentioned that longitudi- 

 nal anastomosing vessels between the intercostals do occur in embryonic 

 pigs. "We have never identified these in early stages, but their presence 

 can be easily demonstrated in older embryos by means of celloidin in- 

 jections that have been converted __ 

 into corrosion preparations. In a 

 pig about six centimeters long, the 

 veins between the ninth and elev- 

 enth intercostals, when thus prepared 

 and viewed from the dorsal side, are 

 represented in Figure 5. From the 

 longitudinal hemiazygos {vn. km'az.) 

 pass off on the left the three inter- 

 costals 9, 10, and 11; on the right, 

 intercostal 11 and intercostal 9, to 

 which intercostal 10 is attached by a 

 longitudinal trunk (vn. az.), which 

 probably represents a part of the 

 once complete azygos. 



Each of the six intercostal veins 

 mentioned gives oflf a short dorsal 

 vessel, which opens into a zigzag longitudinal trunk (vn. Ig.) of the 

 corresponding side of the body. Thus the intercostals of a given side 

 are put into communication with one another by a longitudinal anasto- 

 mosing trunk. The right and left anastomosing trunks are moreover 

 connected transversely, at regular and frequent intervals^ in regions 

 where their inwardly directed angles approach each other, thus pro- 

 ducing a series of more or less hexagonal " islands " bounded by blood- 



FlGUBE 5. 



Fia. 5. The hemiazygos and connected veins from the region of the ninth to 

 the eleventh intercostals in a pig about six centimeters long. Celloidin injection 

 freed by artificial digestion. Dorsal view. X 10. vn. az., azygos vein ; vn. hm'az., 

 hemiazygos vein ; rn. /^., longitudinal anastomosing vein; 9, 10, 11, respectively 

 ninth, tenth, and eleventh intercostal veins of the left side. 



