136 MERISTIC VARIATION. [part r. 



in illustration, concerning the composition of the median and ulnar 

 nerves respectively. 



The median is formed by two heads from the plexus ; into the 

 outer head the Vlth and Vllth spinals enter, while the inner is 

 formed by branches of the Ylllth and IXth, sometimes with the 

 addition of some bundles of the Vllth. The presence of fibres 

 from the Vllth depends on whether the anterior branch of the 

 Vllth bifurcates, or goes wholly to the anterior (outer) cord of the 

 plexus. In order to see whether both Vlllth and IXth contribute 

 to the median, twenty-eight dissections were made, fourteen in 

 infants, fourteen in adults. In one foetus and in one adult no 

 branch from the IXth was found, these being the only exceptions 

 to the rule that both Vlllth and IXth send fibres to the median 

 nerve. The median is then made of the Vlth, Vllth, Vlllth and 

 IXth, but these roots do not send to it a constant proportion. The 

 bundle from the Vlth varies little, that from the Vllth varies 

 considerably, that from the Vlllth is sometimes equal to, some- 

 times smaller, and sometimes larger than the bundle from the IXth. 



The origin of the ulnar nerve was traced in thirty-two cases, 

 fourteen being adults. It was found to arise in four different ways. 

 Most commonly it arose from the Vlllth and IXth : this occurred 

 in twenty-three cases. With the Vlllth and IXth is sometimes 

 combined a strand from the Vllth, as shewn in five cases (four 

 foetal, one adult). In three foetal cases it arose from the Vlllth 

 only, and in one foetal and one adult case from the Vllth and 

 Vlllth. The Vllth is only added to the ulnar in some of those 

 cases in which it gives a branch to the posterior (inner) cord of the 

 plexus. In several cases the branch from the Vlllth was much 

 larger than that from the IXth, but the reverse w 7 as never met 

 with. 



Evidence similar to the above is given respecting other nerves 

 from the brachial plexus. 



From the results of the investigation generally, it appeared that 

 the range of Variation though considerable was not extravagant, 

 and that when parts, usually supplied by some given nerve root, 

 are supplied by some other root, this other root is then either the 

 one anterior or the one posterior to the root from which the supply 

 normally comes. Some muscles seemed to bear definite relations 

 to each other and their nerve supply seemed also " to vary solidly," 

 their nerve supplies remaining the same relatively to each other, 

 though derived from a different root. " The best example of this 

 is in the three muscles which are attached along the inner side of 

 the bicipital groove, the subscapularis, teres major, and latissimus 

 dorsi. The first is usually supplied by the Vth and Vlth, the 

 second by the Vlth, and the last by the Vllth, and however much 

 they may 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 



