The Minute Anatomy of the Brachial Plexus. 



431 



The lesser internal cutaneous is derived in all but very rare excep- 

 tions from the 9th alone. In twenty cases it only once received from 

 the 8th also. 



The posterior branches of the nerve roots unite to form the posterior 

 cord. Each of the four upper roots contributes to the cord, but the 

 9th rarely joins it. Out of forty-five cases where the point is noted, 

 a fibre from the 9th occurs only six times, three times in foetal, three 

 in adult cases. In one of these there was an unusually large root 

 from the 10th nerve. In many adults the branch to the posterior cord 

 can be seen to leave the 8th root before this is joined by the 9th. It 

 may be taken then, as a rule, that the 9th does not contribute to 

 the posterior cord. In the six exceptions the fibre from the 9th was 

 invariably very small, and ran with the 8th, from which it was not 

 isolated. 



The branches of the posterior cord are the three subscapulars, the 

 circumflex, and the musculo- spiral. 



The first subscapular, which serves the subscapularis muscle, is 

 often double, sometimes triple. It never receives from the 7th or 8th 

 root, and is often wholly or partly given off from the 5th and 6th 

 before they are joined by the 7th. Dissection shows that even when 

 given off after the junction the 7th has still no share in it. This 

 rule was invariable in forty-one cases. In twenty-three of these a 

 dissection was made to see to which of the two upper roots the nerve 

 was to be traced. It was found to come from the 5th in eleven cases, 

 from the 6th in three, and from both in nine. I believe that owing 

 to my not being sufficiently mindful that the muscle often receives 

 more than one nerve, I have referred too large a number to the 5th 

 alone. Whether this be so or no, it does not represent the whole 

 supply given to it, for the lower part of the muscle always receives 

 fibres from the second subscapular on its way to the teres major. 



This is a nerve of rather lower origin than the preceding. It was 

 traced three times to the 5th, all foetal cases ; thirteen times to the 

 undivided 6th and 5th; four times to the 6th alone, one being an 

 adult case ; nine times to both 5th and 6th, seven being adult cases ; 

 nine times to both 6th and 7th, seven being adult cases ; and thrice it 

 was formed by a branch from the cord of the upper two roots, and 

 one from the 7th. This shows out of forty-one cases twelve in which 

 the 7th contributed to the nerve, and twenty-nine in which it was 

 excluded. There Were only three in which the supply lay above the 

 6th, and thirteen where it lay wholly below the 5th. 



The question then arose — When the 7th enters the nerve, does it 

 form part of the twigs which supply the subscapularis ? I divided 

 the nerve in three such cases, and found that in each the 7th went 

 entirely to the teres major. 



The lower part of the subscapularis, however, is served, though not 



