The sympathetic is that part of the autonomic system which is connected with the spinal roots from the second thoracic to the second lumbar inclusive (man). Its ganglia are divided by anat omists into the vertebral, those which lie as a double chain on the ventral face of the vertebral column, and those which lie scattered at various distances among the viscera, the pre-vertebral. Langley has shown that there is no essential difference between these except that the vertebral send some of their post-ganglionic fibres into the spinal nerves, whereas the latter send all their fibres to the viscera. The sympathetic sends its post-ganglionic fibres— I. To the muscular coats of the whole of the alimentary canal from the mouth to the rectum; to the glands opening into the canal from the salivary glands in front back to the intestinal glands; to the blood vessels of the whole of the canal from mouth to anus inclusive.
2. To the generative organs, external and internal, and to the muscular coats of the urinary bladder.
3. To the skin; (a) to its blood vessels; (b) to its cutaneous glands; (c) to unstriated muscle in the skin; e.g., the erectors of the hairs.
4. To the iris muscles and blood vessels of the eyeball.
The sympathetic nervous system is sometimes called the vis ceral. It will be seen from what has been said that this term is not well suited in some respects, because the sympathetic supplies many structures which are not visceral. Another objection is that a great deal of important nerve-supply to the viscera is furnished by parts of the autonomic system other than sym pathetic. That the sympathetic does, however, of itself constitute a more or less homogeneous entity is indicated by a curious fact. The substance adrenalin has the property when introduced into the circulation of exciting all over the body just those actions which stimulation of the efferent fibres of the sympathetic causes, and no others. It is possible that when a nerve is stimulated some body at the nerve ending is set free, and this by combining with another chemical substance induces activity in the end organ (gland or muscle). It may be that when a sympathetic nerve is excited adrenalin is set free and combines with some substance which induces activity. (See ADRENALIN ; SUPRA-RENAL Ex TRACT.) The rest of the autonomic system consists of two portions, a cranial and a sacral, so called from their proceeding from cranial and sacral nerve-roots respectively. The cranial portion is sub divided into a part belonging to the mid-brain and a part belong ing to the hind-brain. The ciliary ganglion belonging to the eye ball is the ganglion of the former part, and its fibres innervate the iris and the ciliary muscles. The hind-brain
portion gives pre-ganglionic fibres to the facial (intermedius), glossopharyngeal and vagus nerves ; its post-ganglionic distri bution is to the blood vessels of the mucous membrane of the mouth and throat, to the musculature of the digestive tube from the oesophagus to the colon, to the heart, and to the musculature of the windpipe and lungs.
The sacral part of the autonomic system issues from the spinal cord with the three foremost sacral nerves. Its ganglia are scat tered in the neighbourhood of the pelvic organs, which they inner vate. The distribution of its post-ganglionic fibres is to the ar teries of the rectum, anus and external genitalia and to the muscu lature of the colon, rectum, anus, bladder and external genitalia.
The part played by the sympathetic and the rest of the auto nomic system in the economy of the body is best considered by following broad divisions of organic functions.
Movements of the Digestive Tube.—It is those movements of alimentation not usually within range of our consciousness which the autonomic system regulates and controls. Nor is its control over them apparently essential or very complete. For instance, the pendular and peristaltic movements of the intestine still go forward when all nerves reaching the viscus have been severed. Extirpation of the abdominal sympathetic has not led to obvious disturbance of digestion or nutrition in the dog. It is noteworthy that the sympathetic inhibits contraction of the musculature of the stomach and intestine, while the other, the vagus, portion of the autonomic system excites it.
Action on the Circulation.—The arterial blood supply of most organs is under the control of vasoconstrictor nerves. All vasoconstrictor nerves are sympathetic. Organs to which vaso constrictor nerves are supplied either poorly or not at all are the lungs, heart, liver and probably the skeletal muscles. In some parts the capillary vessels are supplied with sympathetic con strictor nerves. The blood vessels of certain parts of the body have, in addition to vasoconstrictor nerves, nerves which relax their muscular wall, vaso-dilatory nerves. The latter are never furnished by the sympathetic, they are in the mucous membranes and glands at the oral end of the body furnished by the cranial portion of the autonomic system. In regions at the aboral end of the body they are furnished by the sacral portion of the auto nomic system. Elsewhere the vaso-dilatators when present are derived from the nerve-cells of the spinal ganglia (Bayliss).