742 



THE NEEVOUS SYSTEM. 



middle of the clavicle, in the line of the cephalic vein, distally along the front of the shoulder, the 

 lateral border of the arm, forearm and hand, to the lateral border of the thumb. The postaxial 

 border extends from the middle of the axilla along the medial side of the arm (in the line of 

 the basilic vein), the medial side of the forearm and hand, to the medial 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 the inguinal ligament distally along the medial side of the thigh and leg in the 

 line of the great saphenous vein, to the medial side of the great toe. The postaxial border, 

 beginning at the coccyx, extends along the fold of the nates and the lateral border and back 

 of the thigh and leg (in the line of the small saphenous vein) to the lateral 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 medial side and 

 posterior part 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 rami 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 rami 

 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. 



