MORPHOLOGY OF THE 



superior mesenteric artery lying between the jejunum and 

 the colon, but within the mesentery of the former. The 

 caecum, which was previously hidden by the intestinal coils, 

 is readily recognized. 



In Fig. 8 the jejunum and its mesentery have been di- 

 vided at the -|- pictured in Figs. 6 and T, and the colon 

 where it is seen to be bent in the last plate. The stomach 

 and the first and second portions of the duodenum are clearly 

 defined, while the third and fourth portions are still hidden 

 from view. The exact position and course of the intra-ab- 

 dominal portion of the jejunum may now be studied. It 

 begins at the termination of the duodenum, close to the cut 

 superior mesenteric artery, proceeds caudad and then ascends 

 to the umbilicus. A loop is thus formed w^hose convexity 

 faces caudad and to the left. The jejunum then crosses the 

 third portion of the duodenum and ascends parallel to and 

 to the left of its second portion, resting all the time upon its 

 mesentery. This loop of jejunum is absolutely constant, 

 although, as we shall see later, its length depends upon the 

 stage of development of the embryo. 



In order to follow the duodenum throughout its entire 

 course, it is necessary to separate the jejunum from its 

 mesentery and to turn it caudad (Fig. 9). This brings the 

 duodenum into view, and shows clearly the exact position of 

 its four divisions. The first portion is the continuation of 

 the pyloric extremity of the stomach, and rests upon the 

 right central lobe of the liver. The second (D) is the descend- 

 ing portion, the third is represented by its bend, whose con- 

 vexity faces caudad and to the right, while the fourth is the 

 ascending portion which terminates where the jejunum 

 begins, close to and just below the superior mesenteric artery. 



The last three divisions of the duodenum all rest upon 

 the caudate lobe. 



The dissection of this embryo has given us the relations 

 of the intra-abdominal divisions of the digestive tract at that 

 period when the greater proportion of the intestines lie extra- 

 abdominal. Let me briefly recapitulate them. The stomach 



