778 REPORT— 1887. 



identical with, and developed from, tlie dorsal and ventral surfaces of the embryonic 

 limb. 



In the case of the fore limb the following parts are developed from the dorsal 

 surface : the parts in the scapula, the extensor surface of the humerus (the back of 

 the arm), the extensor surJace of the radius and ulna (back of forearm), and 

 back of hand. 



From the ventral surface the folloA^'ing parts arise : the pectoral muscles, the 

 flexor surface of the humerus (front of arm), the flexor surface of the radius and 

 ulna (front of forearm), and the palm of the hand. 



In the case of the hind limb, the region of the buttock, the exfensor surface of 

 the thigh, the front of the leg, and dorsum of the foot are equally continuous with, 

 and developed from, the primitive dorsal surface. The parts on the inner side and 

 back of the thigli (including the adductor and hamstring muscles), the back of the 

 leg, and the sole of the foot represent the ventral surface of the original out- 

 growth. 



Turning again to the nerves, we find that the parts of the fore and hind limbs, 

 which are derived from the originally dorsal surface of the embryonic limb, are 

 supplied by nerves of distribution, which are formed by a combination of dorsal 

 divisions of the nerves formuig the limb-plexus. In the same way those parts 

 derived from the primitive ventral surfaces are supplied by nerves of distribution 

 derived from combinations of the ventral divisions of the nerves entering into 

 the plexus. This holds good without exception throughout the series of animals 

 examined. 



From a consideration (1) of the origin of the nerves and constitution of the 

 plexuses, and {2) of the parts of the limbs supplied by them, viewed in the light of 

 the development of the mammalian limb, the following liypolhesis has been 

 raised : — 



That in a primitive condition of the limb, at an early period of development, 

 the nerves have a simple arrangement and distribution in the simple bud ; the 

 more preaxial nerves supply the preaxial portion of the limb ; the nerve postaxial, 

 the postaxial portion ; while the inferior primary division of each nerve engaged in 

 forming the plexus or in supplying the limb divides into a dorsal and a ventral 

 branch for the supply of the dorsal and ventral surfaces respectively of its own 

 particular part of the embryonic limb. 



The embryonic nerves become differentiated and complicated in their arrange- 

 ment pari jMtssu v,-ith the development of the muscular system, and the changes 

 which take place in the production of the adult condition. 



Positive proof of this hypothesis can only be obtained from an examination of 

 the developing nerves in the embryo. This has been done in the chick, and to a 

 certain extent in mammals also. In a more recent paper I have shown that ( 1 ) at 

 the roots of the limbs the nerves divide into dorsal and ventral branches, which 

 unite respectively with adjacent dorsal and ventral branches, and can be traced to 

 the dorsal and ventral surfaces of the limb; (2) the fn-mation of the plexuses and 

 the passage of the nerves to the distal extremity of the limbs occurs before the dif- 

 ferentiation of the tissues of the limb into muscidar elements. 



Conclusion. — From these data an adequate conception can, I think, be obtained 

 of the fundamental formation of the limb-plexuses. An analysis of these plexuses 

 in mammals has shown that in the production of the ' nerves of distribution' out 

 of the ' nerves of origin ' tAvo events occur : ( 1) The nerves of origin divide into 

 dorsal and ventral trunks. (2) These dorsal and ventral trunks subdivide and 

 unite with the coj-resjMndinf/ subdivisions of adjacent nerves to form the nerves of 

 distribution. The first step has been shown to be the result of an embryonic con- 

 dition — namely, a splitting of the original nerves into trunks for the supply of the 

 dor.sal and ventral surfaces of the embrj'onic limb. 



The second step follows as a necessary result of the changes which occur in the 

 limb in the production of the adult condition. 



The bud which eventually forms the adult limb may be looked upon as the 

 result of the fusion of a certain number (say fi^ e) of jn-imary buds — prolongations 

 from the ventrolateral aspects of certain vertebral somites. Each myotome is sup- 



