Responses of Higher Animals: The Nervous Sysiem - 467 



system, all preganglionic fibers are covered 

 by myelin sheaths, but all postganglionic 

 fibers are nonmyelinated. 



Most of the ganglia of the sympathetic 

 system are lateral ganglia, which lie in close 

 proximity to the roots of the spinal nerves, 

 in the so-called sympathetic chain. This gan- 

 glionated strand of nerve tissue extends along 

 the vertebral column on either side of the 

 body, from the neck, through the thorax, to 

 the abdomen (Fig. 25-16). A few of the sym- 

 pathetic ganglia, such as the coeliac and 

 other collateral ganglia, lie in the abdominal 

 mesentery, some distance from the cord. In 

 either case, however, each sympathetic gan- 

 glion exchanges numerous fibers with the 

 nearest spinal nerve. As may be seen in Fig- 

 ure 25-13, some of these fibers are sensory 

 fibers derived from receptors in the visceral 

 organs; others are preganglionic fibers pass- 

 ing to the lateral or collateral ganglia; while 

 the rest are postganglionic fibers, which re- 

 turn from the lateral ganglion to the spinal 

 nerve. Thus each spinal nerve distributes 

 quite a number of sympathetic fibers to the 

 visceral effectors (arterial, walls, sweat glands, 

 and hair-erection muscles) in each segment 

 of the body wall. From the collateral gan- 

 glia, however, all the postganglionic fibers 

 proceed to the various visceral organs by 

 way .of a number of relatively fine non- 

 myelinated nerves (Fig. 25-16). 



The preganglionic fibers of the parasym- 

 pathetic system are mainly distributed by 

 the vagus nerves. This tenth and largest pair 

 of the cranial nerves arises from the brain 

 stem and passes posteriorly through the neck, 

 thorax, and abdomen, sending branches to 

 the heart, respiratory organs, and to the 

 gastrointestinal tract as far as the small intes- 

 tine. The vagus, however, is not the only 

 channel for transmitting parasympathetic 

 fibers to the visceral effectors. The lens, 

 pupil, and salivary glands receive their para- 

 sympathetic innervation via other cranial 

 nerves (III and VII); while the urogenital 

 organs and the large intestine are supplied 

 by parasympathetic nerves originating from 



the spinal cord in the pelvic region, close to 

 the posterior termination of the cord (Fig. 

 25-16). 



The preganglionic fibers of the vagus and 

 other parasympathetic nerves establish syn- 

 aptic contact with postganglionic fibers in the 

 parasympathetic ganglia, which lie very close 

 to the organs that receive the innervation 

 (Fig. 25-16). Thus all postganglionic neurons 

 in the parasympathetic group possess rela- 

 tively short axons, which terminate in con- 

 tact with effector cells in some particular 

 visceral organ. 



As stated previously (p. 455) the synaptic 

 transmitter, or excitatory substance, of the 

 postganglionic neurons of the parasympa- 

 thetic system is acetylcholine; whereas the 

 equivalent agency in the sympathetic system 

 is noradrenaline. 



BEHAVIOR 



To analyze the full pattern of responses 

 in the individual animal — in short, to study 

 the behavior of the organism as a whole — 

 would be to usurp the subject matter of psy- 

 chology. Consequently the present account 

 of behavior will be limited very strictly to 

 considerations of general biological impor- 

 tance. 



The modern tendency is to explain all 

 discontinuous responses (p. 189) in terms of 

 the intricate interconnections of the nervous 

 system. The nervous system consists essen- 

 tially of a very extensive network of reflex 

 arcs, some simple and others very complex, 

 but all interrelated both anatomically and 

 functionally, via a multitude of association 

 neurons in the brain, spinal cord, and affili- 

 ated ganglia. 



Vertebrate animals are able to perform 

 quite a number of fairly complex, though 

 highly standardized, responses, even without 

 any preliminary training or conditioning. 

 The human infant, for example, immediately 

 begins responding to the manifold stimuli of 

 the environment by performing a number of 

 complex actions, and the child executes these 



