SPINAL CORD AND NERVES 



305 



side of upper arm, radial side of forearm, hand, ulnar side nf 

 forearm, inner side (lower in figure) of upper arm, axilla. The 

 segments 4, 5, 6, 1C are separated from the segment's 8C, ID by 

 a line corresponding to the axis of the limb. There is a similar 

 arrangement in the lower limb. If we consider the embryo- 

 logical development of the limb as shown in the diagram (Fig. 

 181) it is easy to see how this arrangement originated. At a 

 the limb-buds, formed chiefly by a lengthening of the nietameres 



FIG. 181. Diagram of embryonic development of tipper limbs from the metameres UC, !>, 6, 7, 8, 

 ID. (Bolk.) a, 1), (.-, d, e, /show the successive phases of the growth cone of the limU^wing 

 to the lengthening of the metameres destined for the upper limb, and its displacement from 

 the middle line of the body. 



7 and 80, begin to appear ; at b and c these metameres, separated 

 by the axis of the limb, begin to extrude from the median line of 

 the body ; at d and c the nietameres 5 and QC and ID are also 

 displaced from the median line and form part of the cone of 

 growth ; finally at / the arrangement and distribution of the 

 metameres of the limb is the same as those of the adult individual. 

 ( Granting this arrangement of the skin and muscle segments, 

 we next have to consider their constitution and functions separately. 

 With the exception of a few small muscles of the vertebral 

 column, which receive their motor fibres from one ventral root 

 alone, all the other muscles of the human body are supplied by 



VOL. Ill 



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