740 THE yERVOUS SYSTEM 



continuity with them. The centre of tlie neuron is the body of the celL The pro- 

 cesses of the neuron form two sets, the jirotoplasmic, (.lendritic or afferent, and the 

 axis-cylinder process, which is termed the axon or efferent process. The dendritic 

 I)r(^cesses of the neurons of the brain and s})inal cord are usually short, relatively 

 numerous, and profusely branched, but the dendritic or afferent process of a spinal 

 ganglion cell frequently runs a long distance before dividing into its terminal rami- 

 tications. The terminal filaments of the dendrites may be associated either with 

 the arborisations of the axons and dendrites of other cells or with specialised end 

 organs. The axons may be short or long; those of the cord and l)rain terminate 

 either in ramifications round other nerve-cells or in association Avith special end 

 organs. The axons of the spinal ganglia enter the cord and divide into an 

 ascending and a descending branch, both of which give off numerous collaterals, 

 and they terminate in ramifications around other nerve-cells. 



The knowledge of the above facts, and the careful study of the degenerations 

 resulting from the section of nerve-roots and the destruction of nerve-cells, com- 

 bined with repeated observations on the })henomena observable during the 

 development of the nerve-roots and the spinal cord, have led to the forma- 

 tion of the following conclusions concerning the deep connections of the spinal 

 nerve- roots. 



The anterior roots. — The majority of the fibres in the anterior nerve-roots are 

 axons of the cells of the anterior cornua, but some of the smaller fibres are derived 

 from the cells of the posterior cornua, and from the cells of Clarke's colunm and 

 the intermedio-lateral tract, whilst others which pass to the roots from the white 

 columns are believed to be processes of some of the nerve-cells of the cerebellum; 

 therefore if the nerve-cells in the regions mentioned are destroyed, or if the anterior 

 nerve-roots are divided beyond their exits from the cord, the nerve-fibres which 

 form them degenerate to their peripheral terminations. 



The posterior roots. — The fibres of the posterior nerve-roots are processes of 

 the cells in the posterior root ganglia. The fibres in each root external to the 

 ganglion are the afferent or dendritic processes, and the fibres in the internal por- 

 tion of the root are the axons or efferent processes, which pass into the cord; there- 

 fore if the posterior nerve-root is divided external to the ganglion the peripheral 

 portion undergoes degeneration, but if the division is made internal to the ganglion 

 tiien the central part degenerates and the positions of its fibres in the substance of 

 the cord become recognisable. 



Two groups of fibres are distinguishable in each posterior root as it enters the 

 cord, an outer or lateral group of small fibres, and an inner group of large fibres 

 intermixed with some small fibres. The filjres of the outer group enter the cord over 

 the apex of the posterior cornu and turning upwards they form a small column called 

 the marginal bundle or Lissauer's column (fig. 440a), inw^hich they run for a short 

 distance l)efore entering the substantia gclatinosa Rolandi. The inner group passes 

 into the outer part of the posterior column, and it is i>ossible that some of its fibres 

 terminate in Clarke's colunm, but the majority, if not all, divide into two branches, 

 one of wdiich passes upwards and the other downwards; both the main fil»re and its 

 two ternnnal Ijranches give off numerous collateral branches which enter the grey 

 matter and ramify round the cells of Clarke's column and the cells of the posterior 

 and anterior cornua of the same side, and some pass through the grey conmiissure 

 to the grey matter of the opposite side. The terminations of the descending 

 branches of the posterior root-fibres are not definitely known, but they are believed 

 to be in the grey matter of the lower segments of the cord. The ascending liranches 

 j)ass upwards to the medulla, gradually nearing the }K)stero-ni(^dian septum as they 

 ascend, and they terminate in arliorisations round the cells of the nucleus gracilis 

 and the nucleus cuneatus in the medulla o])longata. 



Tracts of the spinal cord. — The fibres which form the white colunms of the 

 spinal cord ar(^ the axons of nerve-cells of the cord, brain, or ganglia, which are 

 passing upwards or downwards in the cord to terminate at a higher or lower level; 

 the former are called ascending fibres, the latter descending, and the degenerations 

 which occur in them are termed ascending and descending degenerations respec- 

 tively. 



Both the ascending and descending fibres are groujH'd in tracts, but it must be 



