THE SYMPATHETIC SYSTEM OF NERVES. 1357 



medullated nerve-fibres derived from both the anterior and the posterior roots of 

 the spinal nerves. The fibres arising from the anterior root are called the splanch- 

 nic efferent fibres and those from the posterior root the splanchnic afferent. Not all 

 of the spinal nerves, however, give off white rami, these strands of communication 

 forming a thoraco-htmbar groiip, from the first or second thoracic to the second 

 or third lumbar nerve inclusive, and a sacral group, derived from the second and 

 third, or third and fourth sacral nerves. The cervical nerves do not give off 

 white rami. 



The splanchnic efferent fibres are the axones of cells located within the 

 lateral horn of the gray matter of the spinal cord. They furnish motor impulses to 

 the unstriped muscle of the vessels and viscera, and secretory ones to the glands of 

 the splanchnic area ; they also convey motor impulses to the heart. Leaving the 

 spinal cord by way of the anterior root, they pass peripherally, enter a white ramus 

 communicans and reach the gangliated cord. One of three courses is then pursued 

 by these fibres : (l) they may end at once by forming arborizations around cells in 

 the ganglion which they first enter, (2) they may pass through this ganglion, thence 

 up or down through an association cord to end around the cells of a node of the 

 gangliated cord above or below the level of entrance, or (3) they may course through 

 the gangliated cord and one of its visceral branches, and terminate in arborizations 

 around the cells of a prevertebral or of a collateral ganglion. It is possible that in 

 some cases the spinal efferents may continue without interruption through the several 

 divisions of its path as far as the terminal ganglia. The path connecting the spinal 

 cord with the involuntary muscle always consists of two fibres, the preganglionic 

 and postganglionic. The latter is the axone of the sympathetic neurone and always 

 forms the last link of the path carrying the stimulus to the involuntary muscle. 



The splanchnic afferent fibres are the sensory fibres of the splanchnic area 

 and consist of the dendrites of cells situated within the intervertebral ganglia on the pos- 

 terior roots of the spinal nerves. Whilst the greater number of these fibres are found 

 in the white rami, a few are thought to be constituents of the gray rami. Beginning 

 in the viscera, they run centrally, without interruption, through the terminal and 

 collateral ganglia, through the gangliated cord and the white (or gray) rami to the 

 spinal nerve, and thence after coming into relation with the cells of the ganglion of 

 the posterior root, they pass by way of the posterior roots into the spinal cord. 



The gray rami communicantes are bundles of axones of sympathetic neu- 

 rones which pass from the gangliated cord to each one of the entire series of spinal 

 nerves. The reason of this generous provision will be evident when the purpose of 

 the communications effected by the gray rami is recalled, namely, to provide sympa- 

 thetic filaments to the outlying muscles and glands by way of the convenient path 

 afforded by the distribution of the somatic nerves. Mingled with the gray fibres, 

 a few of the medullated variety are often encountered ; these are probably partly 

 splanchnic afferent fibres and partly medullated sympathetic fibres. Variation in the 

 origin of the gray rami from the gangliated cord is not uncommon ; they may 

 arise either from a ganglion or from the association cord between two ganglia ; 

 after leaving the gangliated cord, a single ramus may divide and supply two spinal 

 nerves ; or the reverse may happen, two or more rami arising independently and 

 either separately or after fusing, joining a single spinal nerve. 



The further course of the sympathetic fibres, after having joined the spinal nerves 

 by way of the gray rami, is as follows : (i) they may course peripherally along with 

 the anterior or posterior primary divisions of the spinal nerve and convey vasomotor, 

 pilomotor or secretory impulses to the involuntary muscle and glands of the somatic 

 area ; or (2) they may enter the spinal canal by way of the anterior or posterior 

 nerve-roots and be distributed to the spinal meninges, but not to the nervous column. 

 According to Dogiel, it is probable that a small number of axones of sympathetic t 

 neurones enter the root-ganglia of the spinal nerves to end in arborizations around 

 cells of the ganglia. 



The association cords (Fig. 1130) are the longitudinally disposed bundles of 

 fibres comprising the interganglionic portion of the gangliated cord ; they contain both 

 white and gray fibres. The gray ones are the axones of sympathetic neurones which 

 are either passing between adjacent or more remote ganglia, or taking an upward or 



