1 164 



HANDBOOK OF PHYSIOLOGY 



NEHROPHYSIOLOGY II 



ways for impulses to the clie;cstive system. Not all of 

 these areas probably are normally concerned with 

 regulation of digestixe function. The areas found must 

 fill one of two qualifications. They must be concerned 

 with the initiation or performance of one particular 

 function, in which case this hmction must be per- 

 manently damaged by remo\al of the area; or they 

 must be concerned with the modification of digestive 

 function according to information of importance to 

 the system. In the first case electrical stimulation will 

 initiate what is usually a spontaneous event; in the 

 latter, it will imitate the arrival of impulses which 

 lead to a modification of digestive function. In the 

 latter case the incoming impulses can be trom the 

 digestive system or from some other system. If we then 

 establish a series of areas which on stimulation change 

 digestive function, we must also establish the nature 

 of impulses incoming to those areas. 



It is conceivable that we inight acquire some idea 

 about the basic relations between the central nervous 

 system and the digestive system by a study of the 

 phylogenetic development of the relations between 

 the two systems. The information availal:)le is rather 

 scantv, however, as will be seen in the following pages. 



COMPAR.\TIVE PHYSIOLOGY 



The coelenterates have a diffuse network of nerve 

 fibers with intermingled cells. Part of this network is 

 located in the gastrodermis. It is not clear whether 

 the nerve net participates in the extracellular digestion 

 or not, but it is probably connected with the feeding 

 mechanisms (82). These mechanisms reach a high 

 degree of complexity at a stage when transport of the 

 ingested food is still effected through simple flow cili- 

 ary motion or the accessory action of the body wall. 

 Only exceptionally does the nervous system partici- 

 pate in the regulation of ciliary action (64). Rhythmic 

 movements of the gastrointestinal canal appear first 

 in the gastropods and rhythmic secretion after feeding 

 is observed in Helix (54). Carriker (ig) found an 

 abundance of nervous tissue about the stomach region 

 of the snail, Lymnaea stagnalis appressa, indicating 

 possible nervous control. Removal of the verticalis 

 complex in the brain of the cephalopods inhibits food 

 intake for several days according to Sanders & 

 Young (79) but does not affect the digestive process. 

 In annelids the influence of the nervous system on the 

 gastrointestinal tract is remarkably similar to the 

 relations in primates. Excitatory and inhibitory fibers 

 can be demonstrated (66), and VVu (97) found that 



electrical stimulation of the annelid brain inhibited 

 or excited gastrointestinal motility. 



The arthropoda show a great variety in structure of 

 the feeding tube depending on the type of food in- 

 gested, but little evidence has been brought forward 

 indicating nervous control. The insects have innerva- 

 tion to the anterior portion of the gut from the fore- 

 brain, possibly via the subesophageal ganglion. 

 Gersch (38) showed that stimulation of the nervous 

 system in the gnat Corethra caused antiperistaltic 

 movements. The functions of the visceral nervous 

 system in lower animals remain obscure. 



Even if the stomach appears first as a specialized 

 structure in the fishes, there is no evidence that these 

 animals need the central nervous system for gastro- 

 intestinal coordination. The goldfish can perform 

 its gastrointestinal functions without a brain (82). 

 Amphibians and birds lose their feeding reactions 

 after removal of the brain, but whether this is due to 

 lack of initiative or involves coordinating mechanisms 

 is not clear. Decerebrate mammals show definitely a 

 lack of initiati\e, init food placed in the pharynx will 

 be swallowed and adequately digested. 



Even if we have ample morphological evidence that 

 the central nervous system and local nervous net- 

 works enter into relation with the gastrointestinal 

 system at a fairly early stage in development, there 

 is little proof of a constant physiological relationship. 

 For comprehensive reviews of the role of the periph- 

 eral nervous system see Yonge (98) and Colin Nicol 

 (21). 



MASTICATION 



Rhythmic chewing can easily be elicited by elec- 

 trical stimulation of the lower part of the motor cor- 

 tex. Repeated observations in a wide variety of ani- 

 mals have confirmed this observation (31, 65, 78). 

 Experiments by Bechtercw (15) seemed to indicate 

 the presence of a .subcortical center, located in the 

 substantia nigra and responsible for the rhythmicity 

 in chewing. Magoun et al. (60) established that 

 rhvthmic chewing could no longer be obtained on 

 stimulation of subcortical regions after cortical ex- 

 tirpation. Bremer (17) had earlier denied, on theo- 

 retical grounds, the existence of a subcortical station. 

 It was not possible for Rioch to confirm Bremer's 

 observation that the cortical masticatory center was 

 divided into three parts. Instead he postulated that 

 the main function of the cortical masticatory center 

 was 10 inhibit the tone of the jaw openers, and that 



