976 



THE ORGANS OF DIGESTION. 



Mesentery of the Small Intestine and Colon. 



The mesentery is influenced by the increase in length of the small intestine. 

 It becomes fan-shaped, and its length at its insertion into the intestine is many 

 times that of its origin from the lumbar vertebrae ; so it lies in folds and is called 

 a frill. In man after the fourth month, this becomes more complicated by fusions 

 with the posterior body-wall or neighboring viscera. It aifects especially the 

 mesentery of the duodenum and colon. 



The duodenum at first is completely invested Avith peritoneum (Fig. 595) ; 

 later other viscera growing faster are thought to pull it away and appropriate it, 

 and so posteriorly it comes to lie on the posterior wall of the body and becomes an 

 immovable organ (Fig. 603). 



The large intestine possesses a susperisorium attached to the vertebral column 

 and designated the mesocolon. When the twist of the umbilical loop occurred, 

 the transverse colon and its mesocolon were drawn transversely across the duode- 

 num, and a new secondary line of attachment was thus formed. This explains 

 why in the adult we find the body-cavity divided by the transverse colon and 

 transverse mesocolon into chambers, an upper and a lower. In the upper are the 

 stomach, liver, pancreas, spleen and part of duodenum ; in the lower, part of 

 duodenum and the small intestines. The duodenum, in order to get from the 

 upper to the lower space, passes underneath the transverse colon or apparently 

 through the mesocolon (Fig. 596). 



The caecum, ascending and descending colon, also lie with their posterior 

 walls more or less in contact with the body-wall, but sometimes they have a more 



Small intestine 



Mesentery 



Colon 



'Mesocolon 



Kidney 



Eetroperitoneal 



space 

 FIG. 604. Cross section of alimentary canal of Fig. 596 to show mesenteries. (Dexter,) 



or less distinct mesentery as they did originally. The original disposition can be 

 seen by taking a cross section of the alimentary canal in Fig. 596 along the 



Serous Cavity 

 Intestine 



Visceral Layer 

 Parietal Layer 



FIG. 605. Schematic drawing of a cross section of a serous cavity. (Gegenbaur.) 



line X. Each colon and the small intestine are fixed to the aorta and vertebral 

 column by its special mesentery, which allows freedom of motion (Fig. 604). 

 The mesentery is seen to be formed of two layers of peritoneum which surround 

 an intestine completely, except at a posterior line where there is opportunity for 



