a he eee 
MORPHOLOGY OF THE LIMB-PLEXUSES. 621 
Although the mesoblastic material of which the limb bud is composed exhibits in itself no 
segmental divisions at any period of its development, a clear indication of the segmental relations 
of the limbs is obtained from the arrangement of the limb nerves. Taking the nerves which 
supply the limbs as a guide, the segments engaged in the formation of the upper extremity are 
the last five cervical and first two thoracic. The lower extremity is related by its nerves to all 
the lumbar and the first three sacral segments. In each limb, the segments at the preaxial and 
postaxial borders are only partially concerned in limb formation. 
It has been already shown that the somatic branches of the nerves enter the substance of the 
embryonic limb and divide in their course into dorsal and ventral trunks, which supply the 
dorsal and ventral surfaces of the limb bud. The higher nerves supply the preaxial border, the 
lower nerves supply the postaxial border, while the nerves most centrally situated extend furthest 
towards the periphery ot the hmb. 
In order to understand properly the constitution of the limb-plexuses, it is necessary further 
to make a comparison of the surfaces and borders of the embryonic and adult limbs. 
Upper Limb.—-(A) Borders.—The preaxial border of the upper extremity extends from the 
middle of the clavicle, in the line of the cephalic vein, down the front of the shoulder, the outer 
side of the arm, forearm and hand, to the outer border of the thumb. The postaxial border 
extends from the middle of the axilla along the inner side of the arm (in the line of the basilic 
vein), the inner side of the forearm and hand, to the inner border of the little finger. 
(B) Surfaces.—The areas of the limb between these lines, anteriorly and posteriorly, correspond 
to the ventral and dorsal surfaces of the embryonic limb bud. The ventral surface is represented 
by the front of the chest, arm and forearm, and the palm of the hand. The dorsal surface is 
represented by the scapular and deltoid regions, the back of the arm, forearm, and hand. 
Lower Limb.—(A) Borders.—The preaxial border of the lower limb extends from the middle 
of Poupart’s ligament down the inner side of the thigh and leg in the line of the internal 
saphenous vein, to the inner side of the great toe. The postaxial border, beginning at the coccyx, 
extends along the fold of the nates and the outer side and back of the thigh and leg (in the line 
of the external saphenous vein) to the outer border of the foot and little toe. 
(B) Surfaces.—The areas between these lines correspond to the primitive dorsal and ventral 
surfaces of the embryonic limb bud. The unequal amount of rotation in the parts of the lower 
limb obscures the relation of foetal and adult surfaces, which are most easily made out in the 
infantile position of the limbs, with the thighs and knees flexed and the soles of the feet 
inverted. The ventral surface of the embryonic limb is represented by the inner side and 
back of the thigh, the back of the leg, and the sole of the foot. The dorsal surface is represented 
by the front of the thigh and buttock, the front of the leg, and the dorsum of the foot. 
Composition of the Limb-plexuses.—In all mammals the same definite plan underlies the 
constitution of the limb-plexuses. The nerves concerned are the anterior primary divisions of 
certain segmental spinal nerves, which (with certain exceptions at the preaxial and postaxial 
borders) are destined wholly and solely for the innervation of the limb. Each of the anterior 
primary divisions engaged divides into a pair of secondary trunks, named dorsal or posterior, 
ventral or anterior. The dorsal and ventral trunks again subdivide into tertiary trunks, which 
combine with the corresponding subdivisions of neighbouring dorsal and ventral trunks to form 
the nerves of distribution. The combinations of dorsal trunks provide a series of nerves for the 
supply of that part of the limb which is derived from the dorsal surface of the embryonic limb 
bud; the combinations of ventral trunks give rise to nerves of distribution to the regions 
corresponding to its ventral surface. The relation of the nerves derived from the limb-plexuses 
to the areas of the limbs is given in the accompanying tables :— 
I. Upper Limb. 
Origin. Nerves. Distribution. 
Posterior scapular : \ 
ees eee : | Scapular region and 
Fi Sele al r (3) ; shoulder | 
Dorsal trunks |} 2U08Capliar \e, peat Dorsal | 
(Posterior cord)|) Circumflex . t : : eRe A 
(Lesser internal cutaneous (?) ) | ; 2 | 
; _ Arm, inner side 
(Intercosto-humeral (?) ) { 
. * Back of arm, fore- 
Musculo-spiral / v 
4 : \ arm, and hand 
Brachial ats vase’ sie marion Sie ier! Varn aiiens Lantus af f'Gcr02) psf d slbexechel 
Plexus : 
-Nery e to subelay ius | Beontiotichest 
Anterior thoracic (2) f | 
Musculoaaut ite (|.Front of arm and | 
Ventral trunks - pie neous : e all forearm Ventral 
(Outer and |. Lesser internal cutaneous Inner side of arm | om oad 
mner cords) Taftemialiout (| Front of arm and SUrLACE | 
€ Ss , | 
nternal cutaneou \ eae | 
Median . jy Front of forearm and | 
Ulnar \ hand | 
4 Ang ) 
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