128 THE ELEMENTARY NERVOUS SYSTEM 



urally facilitates the progress of the food. Besides peri- 

 stalsis the small intestine exhibits what has been called 

 rhythmic segmentation. This consists of a set of con- 

 strictions whereby a continuous 

 band of food is broken up into a 

 series of masses like a string of 

 beads. Each mass is then sub- 

 divided into two by a new con- 

 striction and the two halves from 

 adjacent masses fuse to form a 

 new mass. By a repetition of this 

 process the food becomes com- 

 pletely mixed with the digestive 

 secretions and is brought abun- 

 dantly in contact with the absorb- 

 ing intestinal walls. 



The walls that carry out these 

 movements (Fig. 37) consist of a 

 mucous epithelium on the inside 

 followed by a submucous layer 

 containing the submucous or 

 Meissner's nerve plexus. This is 

 followed in order by the layer of 

 circular muscle fibers, the myen- 

 teric or Auerbach nerve plexus, and the layer of longitu- 

 dinal muscle fibers, that abuts against the serous layer. 

 In both the submucous and the myenteric plexus the cells 

 present all the characteristics of protoneurones and form 

 a continuous network; these two plexuses may therefore 

 be regarded as true nerve-nets. In addition to this in- 

 trinsic nervous mechanism the small intestine receives 

 nerve fibers from the vagi and the sympathetic chain. 



If these extrinsic nerves for a given part of the small 

 intestine are cut, that part will still exhibit rhythmic con- 



Fio. 37. Diagram of the nerv- 

 ous organization in the intestinal 

 wall of a vertebrate (after Lewis, 

 1910); m, mucous layer; p, sub- 

 mucous plexus; c m, circular 

 muscle; m p, myenteric plexus; 

 I m, longitudinal muscle; , serous 

 layer. 



