The Minute Anatomy of the Brachial Plexus. 435 



second by the 6th, and the last by the 7th, and however much they 

 vary above and below their typical place, they do not change their 

 relations to each other. A similar relation exists between the two 

 supinators and the two radial extensors. These last are sometimes 

 supplied by the 6th, sometimes by the 7th, but they are never in any 

 case placed above the supinators. These are always supplied by the 

 6th alone. The flexor group in the forearm show a similar fixed 

 relation. 



From consideration of the usual regularity of the nerve supply, 

 and of the limits within which alone it varies, I conclude that the 

 nerve roots are not always composed of the same fibres, but that 

 what is in one case the lower bundle of the 5th may be in another 

 the upper of the 6th, and what is now the upper bundle of the 8th 

 will at another time be the lower of the 7th root. This may be 

 expressed as a law. 



LAW I. — Any given fibre may alter its position" relative to the 

 vertebral column, but will maintain its position relative to other fibres. 



It is to confirm laws that exceptions are important. One of my 

 infants showed on both sides a larger root from the 10th or 2nd 

 dorsal nerve than was usual. On the right side the nerve was 

 slightly bigger than the natural, but on the left it was as large 

 as the 9th, and this as large as the 8th, whereas the natural 

 proportion of the 8th to the 9th is about 2 to 1, and the 10th root is 

 a minute fibre only. On the right side the only abnormality in the 

 plexus was that the 9th sent a branch to the musculo-spiral. But on 

 the left the musculo* cutaneous received from the 7th, the median 

 received no 6th, the teres major was supplied by the 7th alone, the 

 circumflex received from the 7th, and the musculo-spiral was formed 

 by the 7th, 8th, and 9th. In the ulnar both 8th and 9th entered the 

 deep branch in the hand. Nevertheless the 4th sent a communication 

 to the 5th, and the suprascapular and subscapular were given off 

 normally. 



It appears that in this case the representation of the muscles in 

 the spinal cord began at the ordinary level, but was more than 

 usually lengthened out, its lowest point sinking so far that the 10th 

 nerve conveyed an excessive proportion of fibres to the plexus. This 

 unusual formation of the peripheral nerves still maintains the relative 

 position of the muscles. 



I do not, however, find that every muscle comprised in the table 

 holds fixed relations with all others. There appear to be groups of 

 muscles which are not intimately related. Thus the rise or fall of 

 the three subscapular nerves does not entail a like movement in the 

 nerves of the forearm. 



I have drawn up from my dissections the following table, giving 

 the usual supply of the muscles of the upper limb. 



VOL. xli. 2 G 



