THE SPINAL COED AS A REFLEX CENTRE 371 



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. 165). These fibres therefore must be derived 

 from cells of the grey matter situated between the levels of the 

 I.I.T. 



ILL. 



V.S. 



FIG. 165. 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 third thoracic 

 nerve. The extent of the lesion is shown in the first figure (III. T). The 

 other sections show the degenerations as revealed three weeks later by 

 Marchi's method. (SHERRINGTON.) 



first and second sections, and they can be traced down the cord 

 through a large number of segments. Analogous methods may be 

 used for tracing the course of the ascending intra-spinal fibres. These 

 mtra-spinal fibres occur in the following situations : 



(1) In the lateral columns immediately outside the grey matter, 

 in the bay between the anterior and posterior horns. 



(2) Close to the grey matter in the anterior basis bundle. 



(3) In the posterior columns, united with the descending branches 

 of the entering posterior roots in the comma tract, and also in the 

 immediate periphery of the cord and abutting on the posterior fissure 

 in the septo-marginal tract. 



(4) Mingled with the fibres of the pyramidal tract. 



