328 PHYSIOLOGY 



CENTRAL PATHS OF SPINAL REFLEXES 



The impulse entering the cord is thus able to affect immediately a number 

 of systems of neurons, namely, cells in the anterior horn, in the posterior 

 horn, in Clarke's column, in the substantia gelatinosa, in the lateral column of 

 the same side of the cord, and the corresponding groups of cells on the oppo- 

 site side of the cord either directly by crossing collaterals or indirectly through 



Ill.T. 



VI. T. 



ILL. 



V.S 



Fio. 164. Cross-sections of spinal cord of a dog, showing the descending nerve- 

 tracts originating in the first three thoracic segments (method of * successive 

 degeneration '). The eighth cervical segment had been excised and 568 days 

 later a cross-cut was made at level of the third thoracic nerve. The extent of the 

 lesion is shown in the first figure (III. T). The other sections show the degenera- 

 tions as revealed three weeks later by Marchi's method. (SHERRINGTON.) 



cells which send their axons across the middle line. Through the ascending 

 and descending fibres of the posterior columns it can also set into action the 

 reflex mechanisms of adjacent segments of the cord. In addition to this 

 direct spread of afferent impulses up and down the cord there is an anatomical 

 basis for a co-ordination between the grey matter of different levels. This 

 co-ordination is effected through the intermediation of the internuncial or 

 intra-spinal fibres which pass up and down the cord from segment to segment. 



The course of the descending fibres may be studied by carrying out a total tran- 

 section of the spinal cord at the sixth cervical vertebra, and six months later, when all 

 the fibres degenerating as a result of the section have disappeared, carrying out a further 

 transection or hemisection a few segments below the first transection. If the animal 

 be killed two or three weeks after the second operation it will be found that a number 

 of fibres in the white matter are degenerated below the second section (Fig. 164). These 

 fibres therefore must be derived from cells of the grey matter situated between the 

 levels of the first and second sections, and they can be traced down the cord through a 



