TRANSACTIONS OF SECTION D. 777 



1. In the arrangement of tlie nerves in the plexuses. 



2. In the distribution of the nerves in the limbs. 



1. From the dissections made and an analysis of results the three following 

 rules have been laid down: — 



(«) The inferior primary divisions of the nerves entering the plexus divide 

 into dorsal (posterior) and ventral (anterior) branches. 



(b) Dorsal branches always combine with dorsal branches, ventral branches 

 with venrral branches, to form the nerves of distribution. 



(e) The essential constitution of a nerve of distribution consequently never 

 varies. A nerve arising from a combination of the dorsal divisions of certam 

 nerves in one animal is never found in another to spring from the ventral 

 divisions of these or any other nerves, and vice versa. 



These three deductions are supported by an examination of both brachial and 

 lumbo-sacral plexuses. 



In the case of the fore limb the inferior primary divisions of the nerves entering 

 the plexus split first of all mto dorsal (posterior) and ventral (anterior) branches ; 

 secondly, the dorsal branches combine to form one set, the ventral branches to form 

 another set, of nerves of distribution. 



In the case of the hind limb also there are two sets of nerves of distribution, 

 one derived from a combination of dorsal or posterior branches, the other from 

 ventral (anterior) branches of certain nerves. 



In neither plexus do ventral divisions combine with dorsal divisions of adjacent 

 nerves ; in neither plexus does a nerve of distribution, derived in one animal from 

 ventral divisions, in another case spring from dorsal divisions, and vice versa. 



The great sciatic nerve maybe divided into three parts: — External popliteal, 

 Internal popliteal, Nerve to hamstrings ; and this I find is the rule in the animals 

 dissected. They lie side by side, more or less closely bound together in a fibrous 

 sheath, but with separate and distinctly different origins. The external popliteal 

 when traced up to the plexus is seen to be formed by a combination of dorsal 

 branches ; the internal popliteal, of which the nerve to the hamstring muscles is to 

 be regarded as a part, is derived from certain ventral branches of the nerves 

 which form the plexus. 



I have shown elsewhere that the same is the case with regard to the human 

 great sciatic nerve, and in birds also the two popliteal nerves are separate up to 

 their origins. 



The nerves derived from the plexuses have, moreover, a fixed and definite 

 relation regarding their position and order of origin, from before backwards. 



i'. Turning now to the distribution in the limbs of the several nerves derived 

 from the plexuses, we find that a similar classification may be made from a con- 

 sideration of the parts supplied. 



The mammalian limb originates as a bud which springs from the ventro-lateral 

 aspect of the body. It is at first directed downwards and outwards, and presents 

 a dorsal, superior, or external surface ; a ventral, inferior, or internal surface ; and 

 a preaxial and a postaxial border. In the centre a formation of cartilage occurs, 

 which afterwards develops into the bones and joints of the limbs. Outside (and 

 above and below) this cartilaginous framework muscular envelopes are formed, 

 giving rise to a double dorsal and a double ventral layer. 



Primitively then the limb presents a dorsal and a ventral surface, each consist- 

 ing of cutaneous and muscular strata, and lying respectively above and below the 

 cartilaginous framework of the limb. These muscular and cutaneous strata have 

 to be supplied with nerves. 



The subsequent changes in the limbs, of elongation, angulation, and rotation, 

 along with the development of the muscular strata into complicated systems, pro- 

 duce important effects in the configuration and structure of the limbs ; so that it 

 is a ditticult matter to make out the relations between the parts of the foetal and 

 adult limbs. 



It is sufficiently clear, however, for our present purpose that, in the case of the 

 fore and hind limbs respectively, the following parts in the adult mammal are 



