162 THE NERVOUS SYSTEM 



the torms of these muscles is an important step in the treat- 

 ment of constipation. 



The External Oblique, the Internal Oblique and the Trans- 

 versus are flat, fleshy muscles, which form the lateral portions of 

 the muscular abdominal wall. Anteriorly, where they are related 

 to the rectus, they form thin aponeurotic sheets, which blend 

 with one another and with those of the opposite side in the 

 linea alba. Their functions are the same as those of the rectus, 

 but, whereas the rectus helps to flex the vertebral column in an 

 antero-posterior plane, the obliques help to produce lateral 

 flexion of the vertebral column. 



It is important to recognise that the muscles of the anterior 

 and lateral abdominal walls are segmental in origin, i.e. the 

 external oblique corresponds to a number of external intercostal 

 muscles fused together into one sheet. These muscles are all 

 supplied by the lower six thoracic and the first lumbar nerves, 

 and each nerve supplies a particular segment. As a result, 

 the muscles are able to contract in segments, and, although 

 this contraction cannot be effected voluntarily, it can be 

 produced reflexly. A localised area of contraction indicates 

 that a " focus of irritation " is present in the particular segment 

 of the spinal medulla which innervates the muscular segment in- 

 volved, and this focus may be due to irritation of the peripheral 

 sympathetic fibres which are associated with that segment. 

 For example, the upper half of the right rectus is innervated 

 by the seventh, eighth and ninth thoracic nerves, and these 

 segments of the spinal medulla not only innervate the muscle 

 but also receive afferent impulses via the sympathetic from the 

 gall-bladder and bile-ducts (p. 264). As a result, cholecystitis 

 is frequently associated with a localised contraction of the 

 upper part of the right rectus muscle. 



Similar contracted areas are often found in gastric ulcer 

 (p. 250), appendicitis (p. 279), renal colic (p. 364) and other 

 abdominal conditions. 



In herpes zoster ; the eruption is found to be limited to the 

 area of sensory supply of a given thoracic nerve, and it therefore 



