98 ZOOLOGY. 



on the dorsal roots form an important distinction from the 

 ventral roots. Of the two branches into which the axis- 

 cylinder of each nerve-cell in these ganglia divides, one 

 passes outwards to terminate in a ramification just under 

 the epidermis, or possibly in some special sense-organ. 



The other branch, on entering the spinal cord, divides into 

 two branches, one running forwards, and the other back- 

 wards ; each of these repeatedly gives off branches which 

 (as well as the main threads) end by an arborization round 

 cells in the grey matter. These latter cells are mostly those 

 which send off axis-cylinders to the ventral root, but there 

 are some whose axis-cylinder-processes run forwards towards 

 the brain. 



These latter fibres, as well as those which come in from 

 every dorsal root, are all afferent with regard to the brain. 

 They form a large part of the white matter : the whole 

 dorsal region of it, and the superficial part of the ventro- 

 lateral region are occupied by fibres conveying impulses 

 brainwards ; while the deeper ventro-lateral region consists 

 of fibres coming from the brain, and bringing impulses to 

 control and modify the reflex activity of the motor cells of 

 the grey matter, among whose dendrons they ramify. 



4. Sympathetic System. The visceral branch of the 

 spinal nerve ends in another ganglion, one of a paired 

 metameric series called the sympathetic system, and united 

 from end to end of the body by nerve-fibres into a con- 

 tinuous nerve- chain. From these ganglia nerve-fibres pass 

 to all parts of the alimentary canal, and to the walls of all 

 blood-vessels (including the heart) in fact, to all the visceral 

 (unstriated) muscles. As already hinted (chap, viii., 8), 

 these fibres are non-medullated. Most of them are efferent, 

 naturally ; there are no sense-organs of such importance in 

 these internal tissues as there are on the surface of the body 

 The function of the sympathetic ganglia seems to be to act 

 as relays or subsidiary governing centres. The fibres from 

 the spinal cord do not pass direct to the muscle-fibres in the 

 wall of the intestine they end by ramifying round cells 

 of the sympathetic ganglia, and these, when stimulated by 

 them, send off the impulse to the muscle-fibres. The ipinal 



