144 MEBJSTIC VARIATION. [part i. 



As regards the sensory nerves in the fore-limb, the following principles 

 were similarly established by dissection in Man. 



1. "Of two spots on the skin, that which is nearer the pre-axial 

 border tends to be supplied by the higher nerve. 



2. "Of two spots in the pre-axial area the lower tends to be 

 supplied by the lower nerve, and of two spots in the post-axial area the 

 lower tends to be supplied by the higher nerve." 



"Thus, if the limb be seen from the front, the two highest nerves 

 on the outer and inner sides respectively are the IVth and Xth. 

 Lower than these the Vth and Vlth take the outer, the IXth and 

 Xth the inner side. Below the elbow the Vlth alone takes the outer, 

 and the IXth alone the inner. In the hand, while the Vlth and IXth 

 continue their positions, the Vllth and Vlllth for the first time join 

 in the supply." Particulars from which this general statement is made 

 are given. Herringham, I.e. p. 439. 



According to subsequent investigations of Sherrington's on the 

 hind-limb, the innervation of the muscles of the posterior aspect of the 

 thigh and leg do not follow the third of Herringham's principles, for in 

 their case the deep layer of muscles is innervated by roots anterior to 

 those which innervate the superficial muscles. The same experiments also, 

 though clearly shewing that the nerve-supply of the skin of the hallux 

 is anterior to that of the 5th digit, gave only equivocal evidence that 

 the same was true of the musculatures of these two digits; and in the 

 thigh the gracilis is not supplied before the vastus externus, whose 

 relation is rather that of ventral to dorsal than of anterior to posterior. 

 Sherrington, C. S., Proc. Roy. Soc, 1892, li. p. 77. 



Recapitulation. 



Some features in the Meristic Variation of the spinal nerves, 

 as illustrated by the foregoing evidence, may be briefly sum- 

 marized. 



In the first place, as might be anticipated from the compound 

 nature of a spinal nerve, when Homoeotic Variation takes place, it 

 does not commonly occur by the transformation of entire nerves, 

 but rather by change in the distribution and functions of parts of 

 nerves. In this respect, therefore, there is a difference between 

 Homceosis in spinal nerves and that in vertebrae, for in the latter, 

 Homceosis is often complete. 



A rough illustration may make this more clear. 



Just as in making up the chapters of a book into volumes, 

 whole chapters may be put into one volume or into the next, and 

 the following chapters renumbered, so it may be with the Varia- 

 tion of vertebras, for these inay belong wholly to one region of the 

 spine or to another. But the nerves are like chapters made up of 

 sections ; particular sections or groups of sections may come in an 

 earlier chapter or in a subsequent one, and the places of those that 

 have been moved on may be filled up consecutively, but it seldom 

 happens that whole chapters are renumbered. Nevertheless it is 

 clear from such a case as that of Brady pus and Choloepus, on the 



