THE LIMBS. 



293 



its axillary or posterior border comes to occupy the position of its 

 spme, while the ilium has to be placed at right angles to the spine 

 and its anterior border rotated outwards until it occupies a posi- 

 tion corresponding to the axillary border of the scapula. 



There is a manifestly spiral twist in the humerus, but it is 

 doubtful if this be in any way due to the torsion which the limb 

 undergoes. 



Nerve Supply of the Limbs.— The Arm.— It is important to 

 note that the limb buds arise from the ventro-lateral aspect of the 

 trunk (Fig. 237) at the junction of the somatopleure with the 

 paraxial mesoblast. Therefore the nerves of the limbs are 

 the nerves of the ventro-lateral zone— the lateral cutaneous 



dorsal spinal muse. :•■ 

 (epi-axial) 



ventral spinal musc.^ 



becomes trapezius, etc. 

 post. prim. diu. 



dorsal limb muse. 



dorsal ramus 



axial structures 

 of limb 



kidney 



uentro-lateral muscles (in 

 somatopleure) 



Fig. 238. — Schematic Section showing the Origin and Arrangement of the Muscles 

 and Nerves of the Limbs. (After Kollmann.) 



branches of the segmental nerves (Fig. 238). The muscles are 

 derived from the ventro-lateral sheet, which gives rise to all 

 the muscles of the body- wall (Fig. 238). As soon as the limb 

 buds appear, bundles of fibres from the anterior and posterior 

 nerve roots of the corresponding body segments enter them and 

 keep time with their growth. The limb nerves are at first so 

 large in comparison with the size of the limb bud that they are 

 crushed together and form a plexus. As they enter the bud, the 



