566 HUMAN ANATOMY. 



THE APPENDICULAR MUSCLES. 



The limbs make their appearance as two pairs of flat buds (Fig. 69), the upper 

 pair being situated in the lower cervical and the lower pair in the lower lumbar and 

 upper sacral regions. Into the buds processes extend from the myotomes of the 

 regions concerned and apparently give rise to the more proximal muscles of the 

 limb, but that they are the source of all the limb musculature is as yet undetermined. 

 The greater mass of this musculature develops from a blastema which occupies the 

 interior of the limb-bud and which cannot at first be distinguished from that which 

 gives rise to the limb skeleton, and whether it represents a condensation of tissue 

 whose fundamental derivation is the myotomes or is a derivative of the ventral 

 mesoderm has not yet been definitely decided. 



However that may be, the limb musculature stands in relation to the anterior 

 divisions of definite spinal nerves, that of the upper limb being supplied by the lower 

 five cervical and the first thoracic nerves and that of the lower limb by the lower four 

 lumbar and upper three sacral nerves, and, furthermore, there is a distribution of 

 these nerves to the muscles which may well be regarded as segmental. It is also 

 worthy of note that in those regions of the trunk in which the Umbs develop the 

 ventral musculature is either very much reduced or, as in the lower limb, practically 

 wanting. 



An examination of the limb muscles shows that they may be regarded as being 

 arranged in a ventral or pre-axial group and a dorsal or post-axial group, and in 

 harmony with this arrangement the nerve-fibres which pass to the muscles arrange 

 themselves in ventral or pre-axial and dorsal or post-axial groups. In the fore-limb 

 the dorsal group is represented by the posterior fasciculus or cord of the brachial 

 plexus, while the ventral one is distributed between the lateral and medial fasciculi. 

 In the lower limb the correct relationships of the two groups of muscles and their 

 nerves are less readily perceivable, owing to the forward rotation which the limb has 

 undergone in order to bring its axis into a plane parallel with that of the sagittal 

 plane of the body, a rotation whicji brings it about that in the adult, excepl in the 

 more proximal portion of the limb, the pre-axial musculature is on the posterior and 

 the post-axial on the anterior surface. The pre-axial nerve-fibres are distributed 

 mainly by the obturator and greater sciatic (internal popliteal) nerves, while the 

 post-axial ones pass to their destinations by way of the anterior crural and greater 

 sciatic (external popliteal) ; and in this connection it is interesting to note that the 

 fibres of the external popliteal or peroneal, if traced to their exit from the spinal 

 foramina, will be found to lie dorsal to those of the internal popliteal or tibial, not- 

 withstanding that the former are supplied to the anterior and the latter to the pos- 

 terior muscles of the leg. 



In this arrangement into pre-axial and post-axial groups there is, accordingly, 

 to be found a clue to the proper understanding of the relations of the nerves to the 

 muscles of the limbs, and a further examination of the two groups will reveal indica- 

 tions of a segmental distribution of the nerves and muscles in each. This arrange- 

 ment may be most satisfactorily understood by means of a diagram (Fig. 555) 

 showing the arrangement of the muscles and nerves in what may be regarded as its 

 fundamental condition. The limb-bud may be regarded as a flat plate whose surfaces 

 are directed dorsally and ventrally. Into the upper portion of this plate the upper- 

 most of the spinal nerves which are associated with it is prolonged, its post-axial 

 and pre-axial fibres passing respectively to either side of its frontal plane, and the 

 succeeding nerves are similarly prolonged into it in succession from above downward. 

 The nerves, however, which •lie along the upper and in the lower limb also along 

 the lower borders of the bud are not prolonged into it quite so far as the others, the 

 free edge of the plate being, as it were, rounded of?, so that it is only the more cen- 

 tral (or upper) nerves of the series that reach that portion of the bud from which 

 the foot (or hand) and digits will be developed. 



It follows from this arrangement that in the adult each spinal nerve concerned 

 supplies a portion of both the pre-axial and post-axial groups of muscles, and, 



