THE PERITONEUM. 977 



vessels, nerves, and lymphatics to enter or return. Two layers of peritoneal con- 

 nective tissue, which lodge vessels, nerves, and lymphatics, constitute a mesentery. 

 The intestine looks as though it had pushed its way into the sac of peritoneum as 

 a finger enters a glove. That comparison is incorrect, for intestine and perito- 

 neum are developed simultaneously. The intestine is not first made and then 

 pushed, as would appear (Fig. 605). 



The intestine and the two layers of peritoneum are formed together. As the 

 intestine recedes, the serous membrane comes from the wall to it in a duplicature. 

 The layer covering the intestine is called visceral; that reflected upon the 

 parietes or wall is the parietal layer, and the passage from one to the other is the 

 mesentery. 



In the adult the small intestine is unchanged, but the mesentery of the right 

 and left colon has been widely separated posteriorly, and it and some of the pari- 

 etal peritoneum have been changed to connective tissue, so each colon becomes 

 fixed and partly extraperitoneal, like the duodenum and pancreas (Fig. 603). 

 The kidneys were always outside, being developed in the retroperitoneal space. 



Summary. 



Separation of the alimentary tube and its mesentery into distinct regions 

 (Hertwig) : 



1. The alimentary canal is originally a straight tube from mouth to anus, 

 near the middle of which the yolk-sac is attached by the vitelline duct (Figs. 586 

 and 587). 



2. The alimentary tube is attached throughout its whole length to the verte- 

 bral column by a narrow dorsal mesentery ; it is also connected with the anterior 

 wall, as far as the umbilicus, by means of a ventral mesentery. 



3 At some distance behind (below) the visceral clefts, the stomach arises as 

 a spindle-shaped enlargement ; its dorsal mesentery is designated as mesogas- 

 triurn. 



4. The portion which follows the stomach grows more rapidly in length than 

 does the trunk, and therefore forms a loop with an upper, descending, narrower 

 arm. which becomes the small intestine, and a lower ascending more capacious 

 arm which produces the large intestine. 



5. The stomach takes on the form of a sac and becomes so turned that its 

 long axis coincides with the transverse axis of the body and that the line of attach- 

 ment of the rnesogastrium. or its greater curvature, which at first was dorsal, 

 comes to lie below or caudad. 



6. The intestinal loop undergoes such a twisting that its lower ascending arm 

 (large intestine) is laid over (ventrad to) the upper descending arm (small intes- 

 tine) from right to left and crosses it near its origin at the stomach. 



7. The twisting of the intestinal loop explains why in the adult the duodenum, 

 as it merges into the jejunum, passes under the transverse colon and through its 

 mesocolon (crossing and crossed parts of the intestine). 



8. The lower arm of the loop, during and after its twisting and crossing of the 

 upper arm, assumes the form of a horseshoe, and permits one to distinguish the 

 c^cum, the colon ascendens, colon transversum, and colon descendens. 



9. Within the space bounded by the horseshoe, the upper arm of the loop 

 becomes folded to form the convolutions of the small intestine. 



10. The mesentery, which is at first common to the whole tube, becomes 

 differentiated into separate regions and adapts itself to the different folds and 

 elongations. It is elongated and here and there undergoes fusion with the peri- 

 toneum of the body-cavity, by means of which it acquires new points of attach- 

 ment, or in certain tracts wholly disappears ; some portions of the intestine are 

 thus deprived of their mesentery (Fig. 614). 



11. The mesentery of the duodenum, and in part that of the colon ascendens 

 and descendens, fuses with the wall of the body. 



62 



