Page 408 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 393

The autonomic nervous system 393

depression on the medial surface of the orbit. This in turn drains through
the nasolacrimal duct into the anterior part of the inferior meatus of the
nose. The nasolacrimal duct, which not uncommonly becomes obstructed,
is about 0.5in (12mm) in length and lies in its own bony canal in the medial
wall of the orbit.

The autonomic nervous system

The nervous system is divided into two great subgroups: the cerebrospinal
system, made up of the brain, spinal cord and the peripheral cranial
and spinal nerves, and the autonomic system (also termed the vegetative,
visceral or involuntary system), comprising the autonomic ganglia and
nerves. Broadly speaking, the cerebrospinal system is concerned with the
responses of the body to the external environment. In contrast, the auto-
nomic system is concerned with the control of the internal environment,
exercised through the innervation of the non-skeletal muscle of the heart,
blood vessels, bronchial tree, gut and the pupils and the secretomotor
supply of many glands, including those of the alimentary tract and its out-
growths, the sweat glands, and, as a rather special example, the suprarenal
medulla.
The two systems should not be regarded as being independent of each
other, for they are linked anatomically and functionally. Anatomically,
autonomic nerve fibres are transmitted in all of the peripheral and some of
the cranial nerves; moreover, the higher connections of the autonomic
system are situated within the spinal cord and brain. Functionally, the two
systems are closely linked within the brain and cord.
The characteristic feature of the autonomic system is that its efferent
nerves emerge as medullated fibres from the brain and spinal cord, are
interrupted in their course by a synapse in a peripheral ganglion and are
then relayed for distribution as fine non-medullated fibres. In this respect
they differ from the cerebrospinal efferent nerves, which pass without
interruption to their terminations (Fig. 275).
The autonomic system is subdivided into the sympathetic and
parasympathetic systems on anatomical, functional, and to a considerable
extent, pharmacological grounds.
Anatomically, the sympathetic nervous system has its motor cell sta-
tions in the lateral grey column of the thoracic and upper two lumbar seg-
ments of the spinal cord. The parasympathetic system is less neatly defined
anatomically since it is divided into a cranial outflow, which passes along
the cranial nerves III, VII, IX and X, and a sacral outflow, with cell stations in
the 2nd, 3rd and sometimes 4th sacral segments of the cord.
Functionally, the sympathetic system is concerned principally with
stress reactions of the body. When this system is stimulated, the pupils
dilate, peripheral blood vessels constrict, the force, rate and oxygen con-
sumption of the heart increase, the bronchial tree dilates, visceral activity is