10 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



particular area or spot may be able to stimulate a wider 

 tract of the spinal cord, and motor impulses from a 

 particular central point may reach more than one muscle, 

 as a given muscular nerve is known to contain fibres from 

 more than one spinal nerve, and a given spot of skin is 

 similarly known to be innervated by more than one spinal 

 nerve. In other words, a limb plexus is a nerve-organ for 

 the supply of the limb as a whole, and for the whole limb, 

 providing for the adequate reception of different impulses 

 and for the proper regulation of muscular action. There is 

 further a morphological significance in the upbuilding of 

 the limb plexuses, in which the segmental arrangement 

 is shown, although faintly. Here then, without going 

 further, we have an instance of the value of segmentation 

 in vertebrate architecture. In elasmobranchs, and in 

 reptiles, it appears certain that the muscular apparatus 

 of the limbs is derived directly from the myotomes. In 

 birds and mammals the segmental character of the limb 

 muscles is obliterated, and they arise in situ from 

 undifferentiated mesoblast. In all vertebrates the nerves 

 are segmental. As in the trunk so in the limbs in their 

 most primitive condition, it is the locomotor mechanism 

 which shows evidence of a segmental origin. But in the 

 mammalian limb, as in the muscular system, so with 

 regard to the nerves, the segmental process is, so to speak, 

 discarded when its work is done. Although the segmental 

 character of the nerves can be traced even in their 

 ultimate distribution to muscles and nerves, yet this 

 feature is partially obliterated, and is forced into a less 

 conspicuous position by the still more fundamental and 

 essential characteristics of the mammalian limb. 



The Value of Segmentation. — In fine, segmentation 

 may be looked upon as a process which stamps the verte- 

 brate organism with ceitain definite features, but is not 



