S TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



neighbouring segmental nerve trunks, a series of nerves of 

 distribution to peripheral areas of skin or muscles, which 

 constitute a motor and sensory mechanism for the whole 

 of the district and for the district as a whole with which 

 the plexus is concerned. This is well illustrated by the 

 arrangement of the limbs, to which attention will be 

 directed later on. 



But it is further remarkable that just as the seg- 

 mental character of the nervous system is most pro- 

 nounced (as might be expected) where the segmental 

 skeleton is most apparent — in the region of the thorax — 

 so in the head region, where osseous segmentation is 

 virtually absent, the segmental nature of the nerves is 

 most doubtful and confused. I do not wish to wander 

 into any fields of speculation about the cranial nerves 

 to-night, but only name them as illustrating the break- 

 down of segmental characters even in the nervous system, 

 and the close association of certain of them (which are 

 most like spinal nerves) with segmental motor mechanisms 

 or organs, e.g., the third, fourth, sixth, and twelfth nerves, 

 which are the motor nerves to muscles derived from the 

 cephalic myotomes. 



The Nature of Limbs. — A study of the structure of 

 the limbs of vertebrates shows the influence that this idea 

 of segmentation has upon morjmologists. Owen, that 

 greatest of modern comparative anatomists, regarded the 

 limbs as offshoots from the costal arches. 



Pinning our faith on segmentation, we ma}' evolve 

 the mammalian limb, in imagination, in the following 

 way : — We may start with the segmental parapodia of the 

 annelid. Amphioxus gives us a further advance with a 

 lateral fold, segmented and therefore representing 

 possibly a fusion of these segmental elements. The 

 evanescent wolffian ridge, with its permanent pectoral 



