1356 HUMAN ANATOMY. 



two principal parts, the gangliated cords and the plexuses, with their associated 

 ganglia. 



The gangliated cord (truncus sympatheticus), one of a symmetrically placed 

 pair of gangliated trunks situated anterior or lateral to the bodies of the vertebrae 

 (Fig. 1133), begins in the head and extends through the neck, thorax and abdo- 

 men to the lower portion of the pelvis. In the head it consists of a plexus of 

 fibres continued up from the neck in an intricate interlacement which follows the 

 internal carotid artery ; and in the pelvis it terminates by the two cords forming a 

 loop or fine inosculation, situated anterior to the coccyx and containing the coccygeal 

 ganglion or ganglion impar. 



The plexuses (plexus sympathetic!) are a series of more or less distinct col- 

 lections of groups of nerve-cells (ganglia) and fibres, situated mainly in the axial line 

 and giving off and receiving fibres connected with the various viscera of the trunk. 

 The component elements of the plexuses and, indeed, of the entire sympathetic 

 system, are the ganglia and the nerve-fibres. 



The ganglia, whilst following a general plan of arrangement as to number, size 

 and position, are subject to wide individual variations and, moreover, where they 

 approach a segmental type, as in the gangliated cord, there is considerable deviation 

 from the arrangement presented by the cerebro-spinal system. A ganglion may or 

 may not be connected with a spinal nerve, but it is always linked by association 

 cords with other ganglia. According to their position, three varieties of ganglia 

 are recognized. One group includes the prevertebral ganglia (g. trunci sym- 

 pathetic! ), those found as nodes in the gangliated cord ; a second variety comprises 

 the collateral or intermediate ganglia (g. plexuum sympatheticorum), which 

 lie either on the peripheral branches of the gangliated cord or in a prevertebral 

 plexus ; whilst to the third set belong the innumerable minute terminal ganglia, 

 composed of nerve-cells which lie at or near the visceral distributions of the sympa- 

 thetic fibres. 



Each ganglion consists of an indefinite number of multipolar neurones, whLh 

 possess one axone and a number of dendrites, the whole cluster of cells being 

 enclosed in an envelope of fibrous tissue. The axone is often medullated in the 

 immediate vicinity of its cell, but usually loses this sheath as it gets farther and 

 farther away from its origin. The course taken by the axone of a prevertebral gang- 

 lion-cell may be one of three : (i) it may pass by means of an association cord into 

 an adjoining prevertebral ganglion, (2) it may proceed as a constituent of a gray 

 ramus communicans to join a spinal nerve or (3) it may follow a splanchnic efferent 

 toward a viscus. 



The nerve-fibres encountered within the sympathetic system include two sets : 

 (a) those derived from the cerebro-spinal system, which are usually medullated, and 

 () the sympathetic fibres proper, for the most part nonmedullated, although as 

 stated above, many of the axones possess a medullary sheath for a short distance 

 beyond their origin from the nerve-cell. This distinction between medullated and 

 nonmedullated fibres is, however, somewhat indefinite, since the medullated spinal 

 fibres often become nonmedullated before terminating, whilst the sympathetic fibres 

 occasionally are medullated throughout their course. 



Kami Communicantes. Where the typical segmental arrangement prevails, 

 as in the thoracic region, each spinal nerve is connected with the adjacent gangliated 

 cord by a pair of short nerve-trunks, known as the rami communicantes ( Fig. 1 129). 

 These are divided into two groups, the white rami and \hz gray rami, a distinction 

 depending primarily upon the difference in the appearance of the strands when seen 

 in the fresh condition ; this distinction, moreover, corresponds with the histological 

 difference above noted white rami appearing so in consequence of the prepon- 

 derance of opaque medullated fibres, and the gray rami possessing the darker tint 

 on accourit of the absence of the refracting myelin coat. The rami communicantes 

 pass directly between the spinal nerves and the gangliated cord, in relation to the 

 latter joining either a ganglion or an association cord between nodes. 



The white rami communicantes are composed almost exclusivefy of the 

 visceral branches of certain of the spinal nerves which use the sympathetic system 

 as the pathway by which they arrive at their destination. They consist of fasciculi of 



