THE LIVER. 



1705 



wise, thus forming the dorsal connections of the transverse mesocolon which exist 

 until fusion takes place between this duplicature and the posterior fold of the omental 

 sac. Since originally all parts of the large gut possess a mesentery, the descending 

 colon and sigmoid are for a time provided with a free mesocolon. In consequence 

 of the increasing bulk of the small intestine the descending colon is pushed not 

 only to the left, but also against the body-wall. The intervening serous surfaces 

 usually disappear behind the gut, which later, therefore, ordinarily possesses a peri- 

 toneal coat only in front and at the sides. In a considerable number of cases, how- 

 ever, this fusion and obliteration do not take place, the mesocolon, although displaced 

 towards the left, then persisting as a free mesentery for this segment of the gut. 

 The fold attached to the sigmoid for a time allows of great mobility ; subsequently 

 this is reduced, although partly retained as the definite mesosigmoid. The rectal 

 segment of the large gut retains its primary sagittal situation, but loses the greater 

 part of its peritoneal coat, becoming attached to the posterior pelvic wall by areolar 

 tissue. 



The ascending colon and caecum, in their downward growth towards the right iliac 

 fossa from the hepatic flexure, carry with them a peritoneal covering. This remains 



Fig. 1439. 



B 



C 



ago. 



Pso- 



Diagrams illustrating formation of greater omentum and omental sac. A shows duodenum and pancreas in 

 mesogastrium unattached ; in B these organs are partly against posterior abdominal wall, posterior wall of lesser 

 peritoneal cavity is still free; in C duodenum and pancreas lie against posterior abdominal wall, posterior wall of 

 omental sac has fused with transverse mesocolon, a, aorta ; d, diaphragm ; /, liver; fl, falciform ligament ; uv, um- 

 bilical vein ; j^, stomach ; tc, transverse colon attached by transverse mesocolon {tmc) ; si, small intestine attached by 

 mesenten,- (>n) \ p, pancreas; ci/t, duodenum; /fis. lesser peritoneal sac; os, omental sac; !o, lesser omentum; g-o, 

 greater omentum ; ago and pgo, its anterior and posterior layers ; _/", fusion between posterior wall of lesser peri- 

 toneal sac and transverse mesocolon. {After Kollmann and Hertwig.) 



unattached over the caecum and appendix, but forms secondary connections where 

 .the ascending colon comes into contact with the abdominal wall ; hence this part of 

 the colon usually possesses a serous coat only anteriorly and laterally. Sometimes, 

 however, obliteration of the serous covering does not take place, the ascending colon 

 being attached by a mesocolon. 



The vermiform appendix being primarily an outgrowth from the large gut, since 

 it represents the morphological apex of the caecum, is completely invested with 

 peritoneum and is without a mesentery. Later the appendicular artery, in its course 

 from the ileo-colic to the appendix, produces a serous fold which stretches from the 

 left layer of the mesentery of the ileum to the caecum and appendix. This fold, the 

 meso-appendix, is, therefore, functionally, but not morphologically, a true mesentery. 



THE LIVER. 



The liver (hepar), the largest gland in the body, is formed of very delicate tissue 

 disposed around the ramifications of the portal vein. It is developed in the anterior 

 mesentery, its mesoblastic elements having a common origin with the diaphragm, 



