1030 THti NERVOUS SYSTEM 



plexuses; (3) still other cells wander even further away from the locality of their 

 origin and invade the very walls of the organs innervated by the sympathetic 

 system. The latter cells occur as numerous small terminal ganglia, most of which 

 are microscopic and which, with the twigs connecting them, form the most 

 peripheral of the sj^mpathetic plexuses. Examples of these are the intrinsic 

 ganglia of the heart and pancreas and the plexuses of Auerhach and Meissner 

 in the walls of the digestive canal. Small, straggling ganglia may be found 

 scattered between these three general groups. In the head, the gangliated trunks 

 and great prevertebral plexuses are represented by the ciliary, sphenopalatine, otic 

 and submaxillaiy ganglia and the plexuses associated with these. The supporting 

 tissue of the sympathetic system accumulates early and is probably all of meso- 

 dermic origin. 



Construction of the sympathetic system. — The sympathetic ganglia may be 

 considered as relays in the pathways for the transmission of impulses from the 

 region in which they arise to the tissues in which they are distributed; the cells 

 composing the ganglia are the cell-bodies of the neurones interposed in the 

 various neurone chains performing this function. A fibre arising from a cell-body 

 in a given ganglion may pass out of the ganglion and proceed directly to its 

 termination upon a smooth muscle-fibre or gland-cell, or it may pass through a 

 connecting trunk to another ganglion and there terminate about and thus trans- 

 mit the impulse to another cell, which, in its turn, may give off the fibre which 

 bears the impulse to the appropriate tissue-element. Fibres arising in given 

 ganglia may pass uninterrupted through other ganglia and proceed to their re- 

 spective destinations. On the other hand, several neurones may be involved in 

 the transmission of a given impulse when sent from a region distant from the tissue 

 to which it is distributed. 



Communication between the central nervous system and the sympathetic is 

 established through both efferent and afferent fibres. In the region of the 

 spinal cord both varieties of fibres pass from one system to the other by way of the 

 rami communicantes, delicate bundles of fibres connecting the nearby sympa- 

 thetic trunk with the respective spinal nerves (fig. 785). 



The efferent fibres of the rami arise in the ventral horn (dorso-lateral cell-group chiefly) of 

 |. I the spinal cord, emerge through the ventral roots, enter the rami, and terminate chiefly about 

 fV-,^ the cells of the nearest sympathetic ganglion; some, however, may pass through or over the 

 / ganglion of the sympathetic cord and terminate about cells in more distant ganglia. Since 

 'S these fibres transmit impulses from the central to the sympathetic system, they are known as 

 ' visceral efferent fibres. They are of smaller size than is the average for the cranio-spinal effer- 

 ent or motor fibres of the ventral root. The visceral afferent fibres are of two varieties: — (1) 

 Peripheral processes of the spinal ganglion-cells which run outward in the nerve-trunk, enter 

 the rami communicantes, pass through the various connecting trunks and terminal rami of the 

 sympathetic and terminate in the tissues supplied by these rami. Such are merely sensory fibres 

 of the cranio-spinal type which collect impulses in the domain of the sympathetic and convey 

 them to the central system by way of the sympathetic nerves and the dorsal roots of the spinal 

 nerves. (2) Afferent sympathetic fibres proper. The actual existence of these has not been 

 long established, and their relative abundance is as yet uncertain. They consist of fibres arising 

 in the sympathetic ganglia which enter the spinal ganglia by way of the rami commnicantes 

 and the cranio-spinal nerve-trunk and terminate in arborisations about the spinal ganglion-cells 

 (fig. 785). The afferent impulses transmitted by these sympathetic fibres are borne into the 

 spinal cord or brain by way of the cranio-spinal fibres of the dorsal roots. These sensory 

 sympathetic fibres must necessarily cither receive the impulses they bear from sympathetic 

 neurones having both peripheral and central processes or they themselves must be axones or 

 central processes of neurones having also processes terminating in the peripheral tissues. 

 Doubtless the variety of visceral afferent fibres first mentioned greatly predominates. 



The thoracic and the lumbar spinal nerves are connected with the sympathetic 

 trunk (gangliated cord) by two rami communicantes. Most of both the visceral 

 efferent and also the visceral afferent fibres (which arise in the spinal ganglia) 

 pass by way of a separate ramus. Both these varieties being of the cranio-spinal 

 type, and, therefore, mcduUated, they give the ramus a white appearance meriting 

 the name white ramus communicans. Fibres of the sympathetic type predomi- 

 nate in the second ramus and tlius it is the grey ramus communicans. The latter 

 consists of: — (1) afferent sympathetic fibres and (2) of sympathetic fibres which 

 join the primary divisions of the spinal nerves and course in them to their allotted 

 tissues (fig. 785). 



In the sacral region, most of the visceral efferent fil)ros pass over the ganglia of the sympa- 

 thetic trunk and terminate in the more peripheral ganglia of the plexuses of this region. This is 



