530 TH?: CELL AND MAMMALIAN HISTOLOGY 



of the wall. Typically there are on the inside smooth muscle fibres 

 running circularly, outside these longitudinal fibres, and between 

 the two a nerve net called Auerbach's plexus. In the small 

 intestine each set of muscle fibres runs strictly in the form of 

 a left-handed screw, one set being of high and the other of low 

 pitch. The two sets are complementary and together lead to the 

 waves of contraction in the intestine called peristalsis. Increases 

 in the circular muscle at various points form sphincters, by 

 contraction of which passage of material from one part of the 

 canal to the next may be checked. Outside the muscular layers is 

 the serous coat, which is continuous with the mesenteries. It 

 consists of connective tissue, with, on its outer surface, a single 

 layer of flat, closely-fitting cells very similar in appearance to 

 pavement epithelium ; this is usually regarded as a condensation 

 of connective tissue, and called a mesothelium. 



LIVER AND PANCREAS 



Besides the relatively simple glands of its surface, the intestine 

 has also given rise to two larger glands, the liver and pancreas, 



L amen cfkver tube.Jji^-^^j /Z] B>ioocl vessel. 



Fig. 412. — A small portion of the liver of a pig in section x c. 300. 



which both arise as diverticula of its epithelial wall. The bile 

 duct remains as an obvious part of the diverticulum, but the liver 

 itself (Fig. 412) has been so much branched that its lumen 

 becomes inconspicuous, and it appears as a solid mass of poly- 

 hedral cells separated by connective tissue into lobules. There is 

 httle resemblance to the epithelium with which it is formally 

 classified. The bile which it produces is secreted into the fine 



