SYMPATHETIC SYSTEM. 99 



What are acquired reflex actions? 



A reflex action which is as strong in an infant as in an adult 

 (the contraction of the pupil in the presence of light) is a primary 

 reflex. Another class of reflex actions require frequent repetition 

 before they are automatically performed, and such actions are called 

 acquired reflex actions, as walking, reading, etc. 



What is automatic action of nerve-centres ? 



There are certain actions which continue, and, while they are 

 closely related to reflex action, do not seem to be reflexes, but to 

 originate in the part. Thus, the peristaltic action of the alimentary 

 tract is not dependent upon the presence of food in the intestines, 

 but may be excited in the absence of food or checked when it is 

 present. The action has been referred to small ganglia and nerve- 

 plexuses found there (Auerbach's and Meissner's), and is consid- 

 ered to originate in the local nerve-centres. This is what is known 

 as automatism or automatic nerve-action. 



What power of inhibition and augmentation of action has the 



nerve-centres ? 



In speaking of the action of the heart it was shown that certain 

 fibres of the vagus nerve check the heart's action, and certain 

 other fibres increase it. This control of the action of organs is 

 not confined to the heart, but similar power of regulation belongs 

 to the nervous centres for many other organs. 



SYMPATHETIC SYSTEM. 

 How is the sympathetic nervous system arranged? 



It is arranged in ganglia and plexuses. It is intimately con- 

 nected with the cerebro-spinal system by communicating branches 

 from each spinal nerve and many of the cranial nerves. 



What is a ganglion? 



It is a collection of gray and white nerve-substance, which is 

 usually oval in outline, and is frequently found in the course of a 

 nerve-trunk. In the sympathetic system the ganglia contain 

 numerous nerve-cells, smaller than those of the brain and spinal 

 cord, and from these cells arise nerve-fibres which distribute them- 

 selves in the plexuses. 



What peculiarities have the fibres of the sympathetic nerve ? 

 They are often smaller than those of the cerebro-spinal system, 



