356 TEXT-BOOK OF EMBRYOLOGY. 



the left kidney being lower than the right, etc. Such cases are not uncommon, 

 two hundred being on record. 



Various theories as to the causes of transposition of the organs have been 

 advanced. In the most plausible of these the anomalous condition is consid- 

 ered as due to the influence of the large veins in the embryo. It seems best, 

 therefore, to consider first the transposition of the heart (dextrocardia, referred 

 to on page 285). 



After the two anlagen unite in the midventral line, the heart constitutes a 

 simple straight tube which lies in a longitudinal direction in the primitive peri- 

 cardial cavity, and which is joined caudally by the two omphalomesenteric 

 veins and cranially by the ventral aortic trunk (p. 224). Normally the left 

 omphalomesenteric vein is the larger and pours a greater quantity of blood into 

 the heart tube than the right. This condition is regarded as the primary factor 

 in the deflection of the tube toward the right side (p. 226; also Fig. 196). If the 

 conditions were reversed, that is, if the right omphalomesenteric vein were the 

 larger and poured the greater quantity of blood into the heart tube, the pri- 

 mary bend of the latter would be toward the left side. Consequently the heart 

 would continue to develop in the transposed position and eventually come to 

 lie on the side opposite to the normal. 



Although dextrocardia is very frequently associated with transposition- of 

 the abdominal organs, it is not necessarily so, for there are cases of the latter in 

 which the heart occupies the normal position. Consequently it seems that 

 further influences must be present to account for transposition of the abdominal 

 organs when the thoracic organs are normal. A number of investigators have 

 emphasized the importance of the influence of the large venous trunks in the 

 abdominal region, especially on the position of the liver and stomach. 



Primarily the omphalomesenteric veins pass cranially through the mesen- 

 tery. Later they form two loops or rings around the duodenum. Then the 

 left half of the upper ring and the right half of the lower disappear, the common 

 venous trunk thus following a spiral course around the duodenum (p. 265; also 

 Fig. 239). This primary relation of the omphalomesenteric vein is retained in 

 the relation of the portal vein to the duodenum. The stomach lies to the left 

 of the portal vein. After the allantoic (placental) circulation is established the 

 umbilical veins pass cranially in the lateral body walls. After the veins come 

 into connection with the liver, the right atrophies and the left increases in size 

 and becomes the single large umbilical vein of later stages (p. 264; also Fig. 240). 

 The right lobe of the liver becomes the larger. 



If, as is maintained by some investigators, the usual position of the stomach 

 and liver is due to the persistence of the left venous trunks, a persistence of the 

 right venous trunks would afford a plausible explanation of the transposition of 

 these organs. _ It is not unreasonable to attribute also the transposition of the 



