SYMPATHETIC NERVOUS SYSTEM. 249 



their origin in the central nervous system — that is, while pre- 

 ganglionic fibers — are all possessed of a small medullated sheath, 

 having a diameter of 1.8 to 4 jx. The postganglionic fiber is in 

 most cases non-meduUated, but this is by no means an invariable 

 rule. In many cases the axons from sympathetic cells possess 

 distinct, although small, myelin sheaths. The autonomic fibers 

 arising from the midbrain, bulb, and sacral cord are distributed 

 to limited parts of the body, while those from the thoracic part 

 of the cord, the great sympathetic system, innervate practically 

 all regions of the body. In their physiological and pharmacological 

 reactions the two groups offer certain contrasts, which have been 

 much emphasized in recent years. For this and other reasons, 

 Langley has suggested a comprehensive classification based on 

 anatomical grounds which is illustrated in the following schema: 



Autonomic. 



Parasympathetic. Sympathetic. Enteric. 



Ocular. Oro-anal. 



Tectal Bulbar Sacral Thoracic 



auto- autonomics. autonomics. autonomics, 



nomics. 



The enteric group comprises the system forming the extensive 

 plexuses of Auerbach and Meissner. We know but little in regard 

 to the anatomical connections of these plexuses with the central 

 nervous system, or the peculiarities of their functional activity; 

 therefore, provisionally, they are set aside as a separate group. 

 The grouping of the remaining autonomic fibers into two great 

 classes — sympathetic and parasympathetic — may or may not 

 come into general use, but the division of the entire system, with 

 the exception of the enteric fibers, into the four varieties of auto- 

 nomics, tectal, bulbar, thoracic, and sacral, according to their 

 point of emergence from the central nervous system, affords a 

 useful basis for physiological description. Unfortunately, in 

 pharmacological and medical literature a variation in this nomen- 

 clature has been adopted by some writers, who use the word 

 autonomic in a narrower sense to designate the fibers grouped 

 under the term "parasympathetic" in the above schema. 



The Nicotin Method. — The course of the autonomic fibers 

 has been traced in many cases to their corresponding sympathetic 



