1178 



THE DIGESTIVE SYSTEM. 



much in size and in the appearance presented by their lining -mucous membrane, 

 that they can be distinguished from one another without difficulty. 



Both the jejunum and ileum are irregularly disposed in the form of crowded 

 loops or coils (Fig. 913) which are connected to the posterior abdominal wall by a 

 great fan -shaped fold of peritoneum, containing their vessels and nerves, and 

 known as the mesentery. Hence the name of intestinum tenue mesenteriale is 

 applied to them. The mesentery is of such a length that the coils are able to move 

 about freely in the abdominal cavity, and consequently the position occupied by 

 any portion of the tube, with the exception of the beginning of the jejunum and 

 the ending of the ileum, can never be stated with certainty. Nevertheless, it may 

 be said that, in general, the jejunum occupies the superior and left portions of the 

 cavity below the stomach, the ileum the inferior and right divisions, its terminal 

 part almost always lying in the pelvis, just before it joins the large gut. 



According to Mall, the most usual arrangement is to find the proximal coils of the jejunum 

 on the left side, and high up. Then the tube crosses the vertebral column below the duodenum, 

 and a few coils are placed 011 the right side. It then crosses to the left side again, and several 

 coils are formed, some of which may descend into the pelvis. Thence it passes again to the 



Two mesenteric lymph glands 

 Mesentery 



*./ xm FJU s>. 



Lymph vessel 



Peritoneal coat^B 



Circular 

 muscular fibres 



Longitudinal muscular fibres 



Fia. 928. A PORTION OF SMALL INTESTINE, WITH MESENTERY AND VESSELS. 

 The peritoneal coat has been removed from the right half, and the two layers of the muscular coat exposed. 



right side, where it is coiled up, and then finally descends into the pelvis. The terminal 

 portion almost always lies in the pelvis, just before it ascends to join the large intestine. 



As the coats of the large and small intestine agree in many particulars, it will 

 be convenient to describe the general structure of the intestines here. Subsequently, 

 any peculiarities of structure in particular regions will be described with the corre- 

 sponding division of the tube. 



STRUCTURE OF THE INTESTINES. 



The wall of the intestines, like that of the stomach, is made up of four coats, which 

 are named from without inwards tunica serosa, tunica muscularis, tela submucosa, and 

 tunica mucosa (Figs. 928 and 929). 



1. Tunica Serosa. The serous coat is formed of peritoneum, and confers on the 

 intestines their smooth arid glossy appearance. It varies in the extent to which it clothes 

 the different divisions of the tube, giving the duodenum, the ascending, descending, and 

 iliac colons, and the rectum only a partial covering ; whilst it clothes the jejunum and 

 ileum, the caecum, the transverse and the pelvic colons completely. The detailed 

 arrangement of this coat will be given with the description of each division of the 

 intestinal tube. 



2. Tunica Muscularis. This consists of unstriped muscle arranged in two layers 



