350 HISTORY OF THE HUMAN BODY 



mals, under the name of jugulars, continue to serve in the 

 same capacity as at first. The changes in these parts are com- 

 paratively slight, and consist mainly in the establishment of 

 two definite branches on either side, the external and internal 

 jugulars, and in the greater development of the subdavians, 

 correlated with that of the fore-limbs, which gains for it an 

 anatomical rank equal to that of the anterior cardinal itself, 

 and suggests the name vena anonyma for that part of the orig- 

 inal anterior cardinal below the entrance of the subclavian, 

 since it appears to be formed by the union of two equal veins. 



These changes of nomenclature, it will be seen, are purely 

 anatomical, and express merely the apparent differences due 

 to change in the caliber or the relative position of the separate 

 portions. There are, however, a few genuine morphological 

 changes in the higher Classes, doubtless rendered necessary 

 by modifications in related parts. The most striking of these 

 occurs in Man and some other mammals and consists of the 

 formation of a connection across the middle line between the 

 right and left jugulars, and the more or less complete atrophy 

 of the right vena anonyma. The left innominate vein thus 

 has to convey all the blood from both sides of head and neck 

 and from both anterior limbs, and as it is greatly increased 

 in size through the assumption of this double task, it early 

 received the name of superior vena cava (anterior or precava), 

 in comparison with the inferior vena cava (posterior or post- 

 cava), which enters the right atrium near it. 



A simple modification takes place in the mammalian pos- 

 terior cardinals. The formation of a transverse connection 

 between the two allows one of them to assume the function 

 of conveying to the heart the blood collected by the metameric 

 branches, and permits the other to sever its anterior connection 

 with the heart. Although there is much variation in this, 

 the more common condition in Man consists of the preserva- 

 tion of the posterior cardinal on the right side in its entirety, 

 into which the other empties by means of the transverse con- 

 necting vein, and is deprived of all direct connection with the 

 heart. The first, or complete vein, was called the azygos, 



