314 Study of the Abdominal and Border-Nerves in Man 



seems, in the main at least;, to be due to local conditions. Not infre- 

 quently the nerves and musculature derived from a given spinal seg- 

 ment have a less extensive development than usual. In such instances, 

 the musculature derived from this segment gives way in part to mus- 

 culature derived from a neighboring segment, and the nerve belonging 

 to the latter covers a territory nearly equal to twice the usual territory, 

 while the nerve belonging to the less developed segment is much re- 

 stricted in distribution. These conditions become clear in a study of 

 the rectus muscle. Similar vaxiations in extent of the cutaneous terri- 

 tory covered by a given spinal nerve, are also frequent. 



The border-nerves may exhibit individual variations, or they may be 

 affected as a group. In the latter case, the variations are intimately 

 associated with the length of the spinal axis and the position of the 

 posterior limb. This association is shown in Table VIII. When the 

 border-nerves spring from the 20th and 21st spinal nerves merely, the 

 condition is, with very few exceptions, found associated with skeletal 

 conditions, Avhich indicate a rediiction in the vertebral axis. In the 

 three exceptions given under Type I, we may assume that the genito- 

 crural nerves were locally affected, and that the condition in these three 

 instances does not indicate an influence exhibited on the border-nerves 

 as a whole. The association of the more distal types of distribution of 

 the border-nerves with extension in the vertebral column is less marked, 

 owing probably to the fact that the relation of the pelvic bones to the 

 vertebrae and the form of the vertebrae were recorded only in the more 

 extreme instances of extension to the amount of a full segment. The 

 frequency and the extent of the variation in the spinal origin of the 

 border-nerves and of the lumbo-sacral plexus, make it important that 

 these factors should be taken into account in making up tables of nerve 

 distribution, like the valuable tables of Head. The marked relation 

 existing between these variations and variations in skeletal conditions 

 is their most noteworthy feature. 



In the valuable paper by Bolk, referred to above (p. 207), he points 

 out that the lumbo-sacral plexus in the anthropoid apes is as a rule in 

 an anterior, or " high," position as compared to man, and that distinct 

 border-nerves are less well developed. In man, however, when the 

 spinal axis is shorter than usual and the lumbo-sacral plexus has an 

 anterior position, we do not, as a rule, find that the border-nerves are 

 reduced in number, although they may arise from but a single spinal 

 nerve. Peripheral courses for nerve development are developed some- 

 what independently of the relation of the position of the limb to the 

 spinal axis. 



