j 90 Basic Structure of Vertebrates 



visceral efferent cells in its gray matter. From these cells pass connector 

 (preganglionic) fibers via "white" rami communicantes to lateral 

 autonomic ganglions. Here synapses are made with exciters whose 

 postganglionic fibers emerge via "gray" rami communicantes and, 

 passing into somatic branches of spinal nerves, follow certain of the 

 branches of these nerves to their ultimate distribution in the skin, 

 where the sympathetic fibers cause contraction of the smooth muscles 

 attached to the bases of hair-follicles. In less time than it takes to 

 read this statement, the hairs over the cat's entire body are "standing 

 on end." Meanwhile other things are happening internally. The wave 

 of sympathetic stimulation contracts the blood-vessels of the skin and 

 the viscera. But the vessels which supply the striated somatic muscu- 

 lature do not contract, nor do those which supply the tissues of the 

 heart (coronary vessels), the lungs, and the central nervous organs. The 

 result is an important redistribution of the blood. Faced by the pros- 

 pect of strenuous somatic muscular activity, blood is withdrawn from 

 regions where, at the moment, it is less needed, and concentrated into 

 the tissues whose organs are likely to be called upon to do hard work. 

 At the same time, the bronchial air-passages are dilated and the re- 

 spiratory movement and heart-rate accelerated. These effects of sympa- 

 thetic stimulation are augmented by endocrine action. The sympa- 

 thetic excites secretory activity of a pair of small glands, the adrenals, 

 situated near the kidneys. The deeper tissue (medulla) of these glands 

 at once pours into the blood a small quantity of epinephrine, 

 ("adrenalin"), which is soon circulating through all the tissues. The 

 physiologic effects of this hormone are similar to those produced by 

 sympathetic stimulation. The combined action of the sympathetic 

 nerves and adrenal secretion is to put the endangered animal on a 

 "war basis." The danger past, cessation of sympathetic stimulation 

 and some counteracting parasympathetic stimulation restore normal 

 internal balance. 



The similarity between the responses to sympathetic stimulation 

 and those induced by presence of epinephrine in the blood is intelligible 

 in the light of the comparatively recent discovery that the activation 

 of an effector by its nerve is a chemical process. The efferent fiber 

 terminates in fine fibrils which are in intimate relation to the muscle- 

 cell. It has been shown that the terminal fibrils of an active sympathetic 

 fiber produce a substance which chemically is closely similar to epineph- 

 rine. This substance has been named "sympathin." It is evidently 

 the sympathin that brings about the reaction of the muscle. Similarly, 

 the terminal fibrils of a parasympathetic nerve produce a quite differ- 

 ent substance, acetylcholine, which induces the response of the 

 effector. In general, sympathetic stimulation causes decrease in secre- 



