174 



Basic Structure of Vertebrates 



PLEXUS OF AUERBACH>- 

 PLEXUS OF MEISSNER* 



SYMPATHETIC 



-PERITONEUM 



Fig. 154. Stereogram of a portion of the small intestine, showing the arrange- 

 ment of sympathetic neurons in the plexuses of Meissner and Auerbach. Motor 

 cells are shown in black, sensory cells with white nuclei. (After Kahn. Courtesy, 

 Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



environment continues to pulsate, but in this case it is difficult to as- 

 certain that the fragment is quite devoid of nervous elements. When 

 a short tubular piece of the intestine is cut out and isolated from all 

 external nervous connection, its characteristic peristalsis (anteropos- 

 terior waves of contraction) continues. In this case, it is certain that 

 nervous tissue is present, for throughout the wall of the intestine (also 

 the stomach and esophagus) and between its inner circular layer and 

 outer longitudinal layer of smooth muscle is a delicate and complex 

 network consisting of nerve-cells and their connecting nerve-fibers, the 

 plexus of Auerbach. A similar myenteric plexus (plexus of Meiss- 

 ner) lies at the internal surface of the layer of circular muscles (Fig. 

 154). It is evident that these plexuses constitute a nervous system 

 which immediately controls the muscular activities of the intestine, 

 presumably by providing numerous local reflex arcs. This control may 

 be exercised in total absence of any connection with the cerebrospinal 

 system. Not only the intestine but also the heart, lungs, and most 

 other hollow visceral organs possess their intrinsic nervous mechanisms 

 consisting of neurons whose cell-bodies are more or less definitely segre- 

 gated into ganglions or gangliated plexuses which lie on the external 

 surface of the organ or may be embedded in its wall. 



Some of the secretory activities involved in digestion of food are 

 directly or indirectly excited by the mere presence of food in the diges- 

 tive tube. Various glands of the body are stimulated to secretory ac- 



