126 A. M. PATERSON. 



3. On the homologies of the spinal nerves. 



Their development shows that the nerves which form the 

 limb plexuses are homologous with the whole nerves in 

 the regions between the limbs, where their arrangement is 

 simplest, and not merely with the lateral branch, as Goodsir 

 supposed. 1 The nerves in both regions first spread out into 

 a ragged bundle. These bundles at a later period arrange 

 themselves into two well-defined cords, the division from the 

 main trunk having the same relative position in both. In the 

 regions between the limbs these trunks represent the lateral 

 and inferior branches ; in the regions of the limbs they are 

 dorsal and ventral in position. 



4. On the development of the limb plexuses. 



I have elsewhere shown 2 that in mammals, and as far as I 

 have been able to make out, the same holds good for birds also, 

 the limb plexus are formed on a definite plan, which is essen- 

 tially the same in all the animals examined, and in relation to 

 both fore and hind limbs. The nerves which form the plexus 

 divide, in the first place, into dorsal and ventral branches. 

 These divisions subdivide, and the secondary cords, whether 

 dorsal or ventral, combine with the cords formed by the 

 division of adjacent (dorsal or ventral) trunks to form the 

 nerves of distribution. Any given nerve to the limb 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 

 formation 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 



1 ' Edinburgh New Philosophical Journal,' New Series, vol. v, Jan., 1857; 

 'Anatomical Memoirs,' vol. 2, p. 201, 1868. 



2 Graduation Thesis, Univ. Edin., 1886, ' On the Spiual Nervous System in 

 Mammalia;' "The Limb Plexuses of Mammals," 'Journal of Anatomy and 

 Physiology,' vol. xxi, 1887, p. 611. 



