THE BRACHIAL PLEXUS 157 



anterior rami of C. 5 and T. i are subjected to stretching 

 much more frequently than the other nerves of the plexus. 



From this summary it may be concluded that existing 

 knowledge of the segmental supply of the upper limb is 

 definite with reference to the fifth cervical and first thoracic 

 segments, but that more data or new methods of examination 

 are required before the muscles associated with the inter- 

 mediate segments can be definitely determined. 



It will be observed, however, that the muscles of the 

 shoulder-girdle and arm are supplied by the upper nerves 

 of the plexus, which take origin from the upper part of 

 the cervical enlargement of the spinal medulla (p. 40), whereas 

 the muscles of the distal part of the limb are supplied by 

 the lower nerves. When acute anterior polio-myelitis affects 

 the cervical enlargement, it is commonly limited to its upper 

 or lower part, and the resulting paralysis is correspondingly 

 of the upper arm or lower arm type. 



SEGMENTAL SENSORY SUPPLY OF THE UPPER LIMB 



Just as certain segments of the spinal medulla are associated 

 with certain muscle groups, so each segment of the spinal 

 medulla is associated with a certain area of skin, and this 

 fact is best appreciated when the mode of development of 

 the limbs is called to mind. 



The limbs arise as lateral buds from the body of the 

 embryo. The upper limb grows out in the lower cervical 

 region at right angles to the long axis of the body, and it 

 contains prolongations of the lower four cervical and the first 

 thoracic segments. It possesses ventral and dorsal surfaces, 

 which are separated by cephalic, or pre-axial, and caudal, or 

 post-axial borders. The anterior rami of the lower four 

 cervical and first thoracic nerves grow out into the bud. The 

 fifth cervical nerve is associated with the pre-axial border, 

 and the first thoracic with the post-axial border, while the 

 intermediate nerves occupy intermediate positions. As the 



