THE ABDOMINAL CAVITY 329 



The valve consists of an upper and a lower segment, which 

 bound a horizontal slit, directed forwards and to the right. 

 The segments are produced by an invagination of the mucous, 

 submucous, and circular muscular coats of the ileum and 

 caecum, but the longitudinal fibres and the serous coat take no 

 part in their formation. Villi are found on the ileal surface of 

 the valve, but the mucous membrane of its caecal surface possesses 

 the characteristic features of large intestine. Two ridges, which 

 are known as \hefrcenula, pass one from each end of the slit and 

 run half-way round the gut. 



The colic valve is arranged so as to prevent the regurgitation 

 of chyme into the ileum, while, at the same time, it offers no 

 hindrance to the passage from the ileum into the caecum. When 

 the caecum is loaded the fraenula become tightly stretched, and, 

 as a result, the valve is securely closed. 



The Caecum is a blind sac about 2\ inches long and 3 inches 

 wide. Normally it occupies the right iliac fossa, and is 

 completely invested by the peritoneum, so that it can be lifted 

 upwards and forwards out of the abdomen. It may sometimes 

 possess a mesentery, or it may not be covered by peritoneum 

 posteriorly, in which case it cannot be withdrawn from the 

 abdomen until it has been mobilised (Fig. 104, i). Owing to the 

 laxity of the peritoneum, the normal range of movement may be 

 considerably increased in some cases (mobile ccecum), and this 

 may predispose to conditions of pericolitis, with symptoms 

 closely resembling appendicitis. 



A freely movable caecum may prolapse into the pelvis and 

 become distended with gas and fluid, which are hindered from 

 escaping by the pressure of the pelvic brim and the superimposed 

 coils of small intestine. As it enlarges, the over-distended 

 caecum drags down the parietal peritoneum, giving rise to painful 

 symptoms. When the gut collapses, on the escape of its 

 contents, the loose peritoneum over it fails to return to its 

 former position owing to lack of elasticity and becomes wrinkled 

 and thrown into vascular folds, which are known as " Jackson's 

 veil." This condition is frequently seen in the region of the 

 caecum and ascending colon during operations for recurrent 

 appendicitis. Its origin has been variously described as 

 congenital, as a result of gradual ptosis of the caecum, and as 

 inflammatory following chronic pericolitis. " Jackson's veil " 

 is very commonly seen in children and is often met with in the 

 adult, but it is rarely to be observed in the cadaver. 



