196 THE NERVOUS SYSTEM 



"focus of irritation " can be determined and this will help 

 to identify the viscus at fault. 



Owing to the limited extent of the spinal medulla, the visceral 

 centres are not placed each in a separate segment, so that two or 

 more visceral centres may occupy the same segment or series of 

 segments. For example, the centre for the stomach is situated in 

 the fifth to the eighth thoracic segments, while that for the liver 

 and gall-bladder occupies the seventh to the tenth thoracic seg- 

 ments. Consequently, when a " focus of irritation " arises in the 

 seventh and eighth thoracic segments, owing to a pathological 

 condition of the gall-bladder, the adjoining cells of the centre for 

 the stomach may be thrown into a condition of increased excit- 

 ability. In this event the afferent impulses which ascend from 

 the stomach after the ingestion of food become altered as they 

 pass through the "focus of irritation." As a result, although 

 the stomach itself is perfectly healthy, food may be vomited 

 immediately after it has been taken. Similarly, a " focus of 

 irritation" in the lower thoracic region may account for 

 frequency of micturition in some cases of appendicitis. 



It must be remembered that the viscera acquire their nerve- 

 supply at an early period of their development and, therefore, 

 those viscera which develop in the median plane are innervated 

 by both sides of the spinal medulla. At a later date, certain 

 of these viscera, e.g. the stomach, take up a permanent position 

 to one side of the median plane, while others, e.g. the coils of 

 small intestine, vary in position from time to time. It would 

 appear that, in the former case, the viscus loses, or neglects to 

 use, the nerves from the opposite side of the spinal medulla, 

 whereas, in the latter case, the innervation from both sides is 

 retained. As a result, referred pains from the stomach, gall- 

 bladder, etc., are not experienced in the median plane, as they 

 are in the case of the small intestine. It is impossible, how- 

 ever, to be dogmatic upon this aspect of the subject, as our 

 present knowledge is very incomplete. 



Viscera which develop to one side of the median plane, e.g. 

 the ureters, are innervated from the same side of the spinal 



