Superficial Veins in the Pig. 457 



forward carrying the blood vessels with it. The internal mam- 

 mary (Vmi) is represented in the figure by a straight line ventral 

 to the thoraco epigastric (Vte). It lies on the mesial side of the 

 muscle layer and has very numerous capillary connections with the 

 thoraco epigastric. Posteriorly it anastomoses with the plexus of 

 the deep epigastric, which runs beneath the superficial epigastric 

 and is not shown in the drawing. The membrana reuniens is still 

 large and well supplied with blood vessels, though it is evident that 

 less blood runs from the body wall to the unbilical vein than formerly. 

 As the internal mammary and thoraco epigastric veins now lie, their 

 paths to the heart are about equal in length, and it is natural there- 

 fore that blood supplied to the outer side of the muscle layer should 

 pass back through the thoraco epigastric and that to the inner, through 

 the internal mammary. It is evident, however, that, as the muscle 

 layer grows forward, it will carry with it the internal mammary. 

 The thoraco epigastric being an axillary vessel must still continue to 

 empty into the axilla and therefore were the lower part of the vessel 

 carried forward, the course of the blood through it, on the outer side 

 of the muscle layer, would become more round about than that 

 through the internal mammary. It is therefore to be expected that 

 the blood, following the path of least resistance, will tend to flow from 

 the lower part of the thoraco epigastric through the numerous con- 

 nections into the internal mammary and that, these vessels enlarg- 

 ing in consequence, the path to the internal mammary will become 

 so easy that practically all the blood from the lower outer body wall 

 will pass that way. This is what proves to be the case. In embryos 

 about 18 or 19 mm. long the thoraco epigastric, as such, reaches its 

 maximal development. Then it drains practically all the outer body 

 wall between the limb buds, back as far as the circulation connected 

 with the spinal cord, while ventrally it receives vessels from the mem- 

 brana and anastomoses very frequently with the superilcial epigastric 

 and with the internal mammary. These anastomoses grow larger so 

 that while the more posterior part of the thoraco epigastric becomes 

 practically continuous with the superficial epigastric, anteriorly it 

 begins to drain largely into the internal mammary. At this latter 

 point a characteristic elbow is usually formed from which, as is 



