254 ELEMENTARY PHYSIOLOGY less. 



the middle line as a lining, the peritoneum, for the 

 whole cavity of the abdomen, and also pass over and round 

 the intestines. The latter thus lie in a fold of the peri- 

 toneum, somewhat as a man lies when slung in a hammock 

 (Fig. 81). 



Other folds of the peritoneum similarly support the 

 other organs in the abdomen. The peritoneum is thus a 

 double bag whose relation to the wall of the abdomen and 

 to the organs in it is similar to that of the pleurae to the 

 walls of the tliorax and the lungs. 



The intestines receive their blood almost directly 

 from the aorta. Their veins carry the blood which 

 has traversed the intestinal capillaries to the portal 

 vein. 



The intestines are m;ide up of four coats : an external 

 thin serotis covering of connective tissue, beneath 

 which is a muscMlar coat connected by a submncoits layer 

 with the inner or mucous coat. 



The muscular coat of the small intestines is made up of 

 two layers ; an outer longitudinal, an inner circular. The 

 circular fibres of any part are able to contract, successively, 

 in such a manner that the upper fibres, or those nearer 

 the stomach, contract before the lower ones, or those 

 nearer the large intestine. It follows from this so-called 

 peristaltic contraction, that the contents of the intestines 

 are constantly being propelled, by succe.ssive and ])ro- 

 gressive narrowing of their calibre, from their upper 

 towards their lower parts. And the same peristaltic 

 movement goes on in the large intestine from the ileo- 

 csecal valve to the anus. 



The submucous layer is composed of loose (areolar) 

 connective tissue, and carries the blood-vessels, nerves, 

 and lymphatics. 



The tube of mucous membrane which forms the inner 

 coat of the small intestines is larger than the muscular tube 

 which surrounds it ; hence to get this greater length of 

 the former stowed away into the shorter length of 



