ON THE FATE OF THE MUSCLE-PLATE. 119 



may be derived from any number of the spinal nerves constituting 

 the plexus, but it is always formed by a combination of either dorsal 

 or ventral nerves. 



This mode of arrangement of the nerves in the plexuses is to be 

 explained by a reference to their embryology, and the mode of 

 development of the different parts of the limbs. The plexus forma- 

 tion is complete, and the nerves of distribution are formed in the 

 embryonic limb long before the appearance of muscles. In the 

 development of the nerves in the limbs the following steps occur. 

 The primitive nerve, in the first place, grows out beyond the lower 

 end of the muscle-plate, and reaches the root of the limb. It there, 

 secondly, spreads out into an irregular series of processes, which pass 

 into the undifferentiated tissue of the limb. Thirdly, these branches, 

 at a later date, arrange themselves in two trunks, one dorsal, the 

 other ventral, which extend still farther into the limb and enclose 

 between them a mass of blastema, from which the cartilaginous 

 basis of the limb is formed. Fourthly, the dorsal and ventral trunks 

 fuse with adjacent dorsal and ventral trunks to form two broad flat 

 bands, from which, still later, the individual nerves as found in the 

 adult are produced. 



The development of the muscular system of the limb, occurring 

 after the formation of the nerves, corresponds with it exactly. The 

 muscles appear first as simple double dorsal and ventral layers, among 

 which the nerves pass as dorsal and ventral bands, formed by the 

 fusion of adjacent dorsal or ventral divisions of the nerves of origin. 

 As these muscular strata lose their simplicity and take on the 

 complex arrangement of the adult, the nerves at the same time 

 become more and more sub-divided, until in the adult the primitive 

 characters of both are considerably masked. 



Still, in the adult mammal, it is evident that the more preaxial 

 nerves in the series supply the more preaxial portions, the postaxial 

 nerves the postaxial portions of the limb,* and the combinations of 

 dorsal divisions and ventral divisions of the nerves are distributed to 

 those muscular and cutaneous areas which are derived respectively 

 from the primitive dorsal and ventral surfaces of the embryonic 

 limb bud.f 



* lI(-rrir)»liam,"Ori th<: Human Brachial Plexus," " Proc. Koy. Soc," Jan., 1887. 

 I "Journal of Anatomy and Physiology," vol. xxi., July, 1887. 



