Four forelimbs of 3 platypuses and 3 forelimbs of 2 echidnas were
examined
to study the precise form of the
brachial plexus and to clarify the structural characteristics of the brachial
plexus in phylogeny. The spinal
components contributing to the plexus (C4–T2) and the formation patterns
of
the 3 trunks of the plexus
were the same as those generally observed in mammals. In the cranial half
of the
brachial plexus from C4, 5
and 6 in monotremes, division into the ventral bundle (lateral cord) and
dorsal
bundle (axillary nerve) is
clear, as in other mammals. However, for monotremes, in the caudal half
of the
plexus from C7 and T1
(+T2) and the nerves arising from the caudal plexus there is no definite
division
into the ventral and dorsal
bundles, which distribute to the flexor and extensor parts of the forelimbs,
respectively. The lower trunk of
the monotreme brachial plexus forms a cord which contains both ventral
and dorsal
components. This
characteristic diverges from the generally accepted idea that the tetrapod
limb
plexus is divided clearly into 2
layers: a dorsal layer for extensors and a ventral layer for flexors of
the limb.
Considering the incomplete
dorsoventral division of forelimb nerves in some reptiles and urodeles,
the caudal
half of the monotreme
brachial plexus has characteristics in common with those of lower tetrapods.