ANATOMICAL PROBLEMS BEARING UPON EVOLUTION. 9 



and pelvic limbs in higher vertebrates, may represent the 

 vestige of this lateral fold : the limbs of elasmobranchs 

 and other fishes, with their more numerous nerves, repre- 

 senting a lower stage of evolution than the limbs of 

 reptiles, birds, and mammals, in which fewer nerves (and 

 therefore segments) are engaged. 



Unhappily this theory lacks adequate proof. In the 

 mammalian limb at least the only strictly segmental 

 structures engaged are the nerves, and they, as pointed 

 out already, lose their segmental character for the most 

 part owing to the formation of the limb -plexuses. 



The mammalian limb is formed as a flap Cr fold of 

 undifferentiated mesoblast, in which the skeleton forma- 

 tion arises as a cartilaginous core, surrounded by strata 

 of cells from which the muscles of the limb are produced. 

 The vessels arise in situ, and although ultimately the 

 structure of the limb, as far as its muscular system is 

 concerned, is related (indirectly) to the segmental muscle- 

 plates, the nerves are our only guides to the segmental 

 character of the limbs, as Groodsir first pointed out. 



There have been many efforts to trace these seg- 

 mental nerves through the plexuses and through the 

 limbs. I do not propose to enlarge to-night upon the 

 work that has been done by anatomists, physiologists and 

 clinicians in this field ; but we have now a fairly clear 

 picture of the significance of a limb plexus. We know 

 that a plexus does not mean confusion of distribution. It 

 can be made out by careful dissection, by experiment and 

 by clinical observation that the chief occurrence in the 

 formation of a plexus is (1) the division of a series of 

 spinal nerves into subordinate cords, and (2) the union of 

 these subordinate cords (large or small) with one another, 

 so as to produce a series of nerves for the innervation of 

 I he limb in such a way that different impulses from a 



