668 THE NERVOUS SYSTEM 



synapse takes place in the cardiac ganglion. In short, the second group 

 of ganglia described in the introductory paragraph on the autonomic 

 system belongs to nerve mechanisms of this special group. 



The efferent nerve groups of the cranial, thoracic and sacral regions 

 are all similar in one functional regard, viz., there is no voluntary control 

 of the functions which they influence. These nerves as a whole innervate 

 the eyes, salivary glands, blood vessels, heart, bronchial tubes, stomach, in- 

 testine, colon, liver, pancreas, kidney, urinary bladder and genital organs, 

 all so-called involuntary organs and mechanisms. All of these organs 

 have a double innervation as shown in detail in figure 418. This simi- 

 larity of neurone pathway and of involuntary functional control led Lang- 

 ley to reclassify the whole nerve outflow under the name, the autonomic 

 system. 



It will be seen that each visceral organ receives groups of fibers from 

 two quite different sections of the brain or cord. However, physiologically 

 these two sources of nerves provide the organ with diametrically opposed 

 nerve influences controlling or regulating its function. For example, 

 in the iris of the eye one set of fibers stimulates constriction of the pupil 

 and the other dilation. One set of fibers, the vagus, inhibits the heart, 

 the other set, the augmentors, accelerates it, etc. Autonomic fibers from 

 the thoracico-lumbar cord in general have exactly the opposite influence on 

 function which the fibers from the brain stem and the sacral cord exert. 

 The similarity of functional differentiation is revealed also by the similarity 

 of action of drugs on the cranial and sacral nerves, as contrasted with 

 the thoracic and lumbar nerves. This group of autonomic nerves may 

 therefore be subclassed into cranio-sacral and thoracico-lumbar autonomies. 

 Langley originally classified the oculomotor and medullary outflow, 

 and the sacral outflow as parasympathetic, in contrast with the thoracic, 

 for which he retains the name sympathetic. It seems better to use the 

 more logical classification in the functional grouping as outlined. 



To the more diffuse innervation of the walls of hollow organs, such as the 

 stomach and intestine, Langley gave the name enteric system, see page 669. 

 The best known portion of the enteric system consists of the myenteric 

 plexus (Auerbach) and the submucous plexus (Meissner). These diffuse 

 nets of ganglia are the only groups of nerve cells outside of the central 

 axis in the mammalia known to coordinate reflexes. The system is 

 phylogenetically an older nerve differentiation. The enteric ganglia are 

 also pathways for extrinsic autonomic nerves from the cerebrospinal axis 

 in which it is assumed with little or no proof that the enteric ganglion cells 

 are postganglionic in their homologies. Portions of the urogenital system, 

 tubules and vesicles, possess a similar diffuse nerve mechanism. The 

 whole schema is tabulated below. This classification is now rather 



