322 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



What we mean by saying that the intestines are outside 

 of the peritoneal cavity is shown in Fig. 206. 



Fig. 206. — Diagram showing the relation of the intestines to the abdominal 

 cavity : z, intestine ; vc, visceral cavity ; per, peritonaeum ; m, muscular 

 coat ; ep, epithelial coat ; sp, spinal column ; ao, aorta ; spc, spinal cord. 

 (After Wiedersheim. ) 



Coats of the Intestines. — Like the stomach, the 

 intestines have three coats — the outer, peritoneal ; the 

 middle, muscular; and the inner, epithelial. In other 

 words, it is a hollow, muscular tube, invested with the 

 peritoneal coat and lined with epithelial or mucous mem- 

 brane. The peritoneal coat gives smoothness, the mus- 

 cular coat does the mechanical work, and the epithelial 

 the chemical work of digestion. 



Mechanical Work. — The muscular coat consists 

 mainly of two sheets of parallel fibers. In the outer 

 one the fibers run lengthwise, in the inner ringwise. 

 Under the contraction of these the intestines may be 

 seen to squirm wormlike from side to side, and light 

 waves of contraction may be seen running always in 

 one direction — downward. This is the peristaltic action 

 of the intestines. The waves must be light, otherwise 



