MESENTERY. 211 



on the surfaces of the muscle layers but also in their interior. Their 

 abundance is directly proportional to the thickness of the musculature. 



The serosa consists of connective tissue which is covered with meso- 

 thelium except along the line of attachment between the intestine and its 

 mesentery. As shown in the diagram, Fig. 237, the mesentery is a thin 

 layer of connective tissue bounded on either side by mesothelium, which 

 serves to suspend the intestine from the median dorsal line of the body 

 cavity. It is present unless adhesions occurring in the course of develop- 

 ment have destroyed it, and in the small intestine such adhesions involve 

 only a part of the duodenum. At the root of the mesentery (the portion 

 attached to the trunk of the body) the mesothelium extends laterally and 

 with the underlying connective tissue forms the parietal peritonaeum. 

 The tunica serosa of the intestine and the lateral parts of the mesentery 

 constitute the visceral peritonaeum (this term being applied especially 

 to the former). The mesothelium of the entire peritonaeum consists of 

 flat, polygonal cells shown in surface view in Fig. 238. The outer por- 

 tions of the cells fit closely, but the deeper parts, containing the nuclei, 

 are joined by intercellular bridges. Beneath the epithelium there is 

 fibrillar connective tissue containing abundant elastic networks parallel 

 with the surface, -and having plasma cells and other free forms in its 

 meshes. These cells are found especially along the blood vessels. 

 The connective tissue layer is denser in the parietal than in the visceral 

 peritonaeum. In places where the peritonaeum is freely movable there 

 is a subserous layer of loose fatty tissue, but there is no distinct subse- 

 rous layer in the intestine. The mesothelial layers on the opposite sides 

 of the mesentery are so close together that they may both be seen in a sur- 

 face preparation by changing the focus, or even simultaneously as at X in 

 Fig. 238. The connective tissue between them is thin except where it 

 surrounds the larger blood and lymphatic vessels and nerves which pass 

 through the mesentery to and from the intestine. 



Blood vessels of the small intestine. The arteries pass from the mesen- 

 tery into the serosa in which their main branches tend to encircle the 

 intestine. Smaller branches from these pass across the muscle layers to 

 the submucosa in which they subdivide freely (Fig. 239, A). In crossing 

 the muscle layer they send out branches in the intermuscular connective 

 tissue. These and the arteries of the serosa and submucosa supply the 

 capillary networks found among the muscle fibers. The capillaries are 

 mostly parallel with the muscles. From the submucosa the arteries invade 

 the mucosa forming an irregular capillary network about the glands, and 

 sending larger terminal branches into the villi. There is usually a single 

 artery for a villus and it has been described as near the center with the veins 



