PAKKEK AND TOZIER : POSTCARDINAL VEINS IN SWINE. 137 



by the azygos vein was transferred by transverse connecting vessels to 

 the hemiazygos, by which it was carried to the heart. In the specimen 

 from which the reconstruction shown in Figure 3 was made, three such 

 transverse connections were found. In a specimen fifty-five millimeters 

 long, studied by injection, five such vessels occurred, and these were so 

 placed that their ends were opposite the mouths of the newly forming 

 intercostal veins. 



The most striking peculiarity of the stage illustrated by Figure 3 is its 

 lack of symmetry. In the earlier conditions described these veins have 

 been bilaterally symmetrical ; but with the loss of connection between 

 the azygos and the right Cuvierian duct this symmetry disappears, and 

 a connection with the heart is retained only through the left side. In 

 this respect the pig and probably all ruminants differ from other mam- 

 mals, in which as a rule the azygos, not the hemiazygos, retains its 

 original connection with the heart. 



The further changes that the azygos and hemiazygos undergo may 

 be seen in pigs ranging in length from seven to twenty centimeters. 

 The chief features of these changes consist in the further reduction of 

 the azygos, together with the retention of the transverse connecting 

 vessels, by which the right intercostals are brought to connect directly 

 with the hemiazygos. Depending upon the way in which the azygos is 

 reduced, three types can be distinguished. These are illustrated in 

 Figure 4. 



In the first type (Fig. 4, A) the hemiazygos reaches from the heart 

 posteriorly to the eleventh intercostal space, receiving in its course the 

 intercostal veins on the right from the sixth to the eleventh, and on the 

 left from the fifth to the eleventh. Posterior to the eleventh intercostals 

 two longitudinal veins appear, which are of about equal size, and extend 

 posteriorly two segments farther, receiving the twelfth and thirteenth 

 intercostals. Of these the left one {vn. km'az.) obviously represents the 

 posterior continuation of the hemiazygos, the right one {vn. az.) the last 

 remnant of the azygos, which in the region of the eleventh intercostal 

 still retains its transverse connection with the hemiazygos. 



In the second type (Fig. 4, B) the hemiazygos {vn. hrrCaz.) extends 

 as the predominant vessel from the heart to the fourteenth intercostal 

 space. The azygos is entirely suppressed, except for a small part run- 

 ning from the twelfth to the thirteeth intercostal and possessing at its 

 two ends transverse connections with the hemiazygos. 



In the third and last type (Fig. 4, C) the hemiazygos is a well devel- 

 oped trunk from the heart to the ninth intercostal, beyond which the 



