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The cervical sympathetic trunk--a new hypothesis.

There is a sympathetic ganglion, in relation to most if not all thoracic and lumbar segments whereas there are only 3 or 4 sympathetic ganglia in the cervical region. Thus it may be inferred that sympathetic ganglia are not directly related to spinal nerves. Therefore, the hypothesis is put forward that sympathetic ganglia are associated chiefly with intersegmental vessels such as the intercostal and lumbar arteries and that the differences seen in the neck region are due to the disappearance of most of the cervical intersegmental arteries and the subsequent modifications that follow during development. This results in the fusion of the upper 4 cervical ganglia to form the superior cervical sympathetic ganglion in relation to the developing external carotid artery which seems to provide the necessary inductive stimulus. Furthermore, the antero-inferior migration of the heart, its corresponding arch arteries and the dorsal aorta bring about the formation of the ansa subclavia around the subclavian artery and the positioning of the stellate ganglion behind the vertebral artery.