Superticial Veins in the Pig. 459 



shown in Fig. 9, vessels pass to the internal mammary. Only one 

 of these persists in older stages. There are all manner of variations 

 as to the proportion of the thoraco epigastric that is left draining into 

 the axilla. Sometimes almost the whole vessel is taken over com- 

 pletely and sometimes, as in Figure 9, a considerable portion is left. 

 This figure shows very well the transition stage of the thoraco epi- 

 gastric. The vessel is seen passing on along the edge of the muscle 

 layer to a point where a fan-shaped plexus of vessels conceals its 

 anastomoses with the internal mammary. Here it swings in a long 

 loop back to its axillary connection. This loop is, however, noticably 

 smaller than the vessel below the turn, and it is evident that a good 

 deal of the blood has already been deflected into the internal mam- 

 mary. This connection with the internal mammary is jjlainly shown 

 in the next figure. (Fig. 10). It also shows how far the internal 

 mammary has developed (AmiVvmi), that it is now a double vein 

 with the artery running between, receiving anterior intercostal veins, 

 which anastomose with the intercostals proper. It also receives a 

 great many fine, anastomosing vessels, which have been dissected 

 away from the upper part of the membrana reunions. The mem- 

 brana is still of considerable size but the blood vessels are of a very 

 feeble type compared with the earlier ones, and drain almost entirely 

 back to the body wall through the superficial and deep vessels. As 

 the membrana grows smaller and smaller, the blood vessels on the 

 surface atrophy until only those along the edge of the advancing 

 muscle layer are left visible. This is shown in figure 11, which is 

 from an embryo 3 cm. long. This embryo illustrates chiefly, how- 

 ever, a case where almost all the thoraco epigastric along with the 

 superficial epigastric with which it forms a continuous vessel, drains 

 into the internal mammary. The internal mammary has been carried 

 forward, while the stump of the thoraco epigastric has been left well 

 back in the axilla. This is a very typical case. The chief variation 

 being, as I said before, in the proportion of the thoraco epigastric 

 taken over to the internal mammary. As the membrana continues 

 to recede, and the muscle layer advance, the internal mammary ves- 

 sels are carried nearer together and along with them the superficial 

 veins, so that ultimately we find the transferred portion of the 



