ON THE MORPHOLOGICAL CONSTITUTION OF LIMBS. 201 



plagiostomes, uniphibians, and reptiles, the priuciple which 

 lies at the basis of the morpliology of liinLs. The view which 

 this iiiquiiy has induced me to take of this subject I shall, in 

 conchisiou, state very briefly. 



1. A limb does iKjt necessarily derive its elements from 

 one somatome — about fifty segments of tlie trunk appear to 

 contribute towards the stmcture of the great pectoral fin in 

 the ray. 



2. The nervous elements of the limbs appear, as in other 

 parts of the vertebrate animal, to indicate most distinctly the 

 moqjhological constitution of the sclerous elements. About 

 fifty spinal nerves contribute the greater part of their luenial 

 divisions to the pectoral fin of the ray ; and there are about 

 one hundred fin-rays — a pair of fin-rays to each nerve, and 

 derived from each sclerotome. This correspondence does not 

 apparently exist between the fin-rays and nerves of the 

 osseous fish; but it may Ijc fairly assumed that when we 

 have detected the developmental circumstances which induce 

 the attachment of the pectoral girdle of the osseous fish to its 

 cranium, as well as those peculiarities exhibited by its 

 anterior trunk sclerotomes, this discrepancy will be explained. 

 A more careful analysis than we yet possess of the number of 

 spinal nerves which supply branches to the limbs of the 

 higher Vertebrata is still a desideratum in this department of 

 the subject ; but it appears to be extremely probable, that in 

 the ^Mammalia at least five spinal nerves transmit filaments 

 to the five distal divisions of the limb. It would appear, too, 

 that, notwithstanding their plexiform arrangement at the 

 attached end of the limb, the gi'eater number of the filaments 

 of each nerve reach their own morphological district at the 

 distal pait of tlu> limb. Tlir iwlial and the ulnar nerves 

 are formed principally by the upper and lower roots of the 

 human brachial ])lexus — that is, from the nerves of the 

 upper and lower primordial scgment.s with which the embryo 



