SECONDAKY DEGENERATION OF TROPHIC CENTRES. 



635 



after a time, the direct and crossed pyramidal tracts (fig. 455, 1, 1', 2, 2') degenerate 

 downwards, i.e., they undergo descending secondary degeneration, because 

 are cut off from their nutritive 

 or trophic centres, which are 

 situated above in the pyramidal 

 cells of the motor areas of the 

 brain ( 378). The trophic 

 centre for the fibres of the 

 anterior root lies in the multi- 

 polar nerve-cells of the anterior 

 cornu of the grey matter of the 

 cord. After section of the 

 spinal cord, Goll's column and 

 the direct cerebellar tracts de- 

 generate upivards, i.e., they 

 undergo ascending secondary 

 degeneration. If the posterior 

 columns even be divided, Goll's 

 column degenerates upwards 



towards the medulla oblongata, Yi. 455. 



and the degeneration ends in Transverse section of the spinal cord, showing the secondary 

 the posterior pyramidal nucleus degeneration tracts. AR, anterior, TR, posterior root ; 

 or clava. The same .result 1, 1' (CPT), region of the crossed pyramidal tract ; 2, 2' 

 occurs if the posterior nerve- (^T). direct pyramidal tract; PEC, postero-external 



o ,, r j -u column ; LC, lateral column, 



roots 01 the cauda equina be 



injured. Hence, fibres seem to pass from the posterior root into these columns, and 

 the nerve-cells in the clava must also have an important relation to these nerve-fibres 

 and the parts whence they are derived. The postero-external column remains un- 

 degenerated, so that there is a very sharp distinction between the two parts of the 

 posterior column. As Goll's column degenerates upwards, it points to its fibres 

 conducting impulses in a centripetal direction, and to the nutritive centre for its 

 nerve-fibres being below. The trophic centre is probably in the spinal ganglion of 

 the posterior root.] 



[If the cord be divided above the junction of the dorsal and lumbar regions, the 

 direct cerebellar tract undergoes ascending degeneration, which extends through 

 the restiform body to the cerebellum. Its trophic centre is probably in the cells of 

 Clarke's column.] Those fibres of the spinal cord which do not degenerate after 

 section of the cord, especially numerous in the lateral and anterior columns 

 [anterior ground bundle, the anterior and lateral mixed zones of the lateral column, 

 and the postero-external part of the posterior column], are commissural in function, 

 connecting ganglionic cells with each other, and are, therefore, provided with a 

 trophic centre at both ends. 



Time of Development. With regard to the time of development of the individual systems, 

 Flechsig finds that the first formed paths are those between the periphery and the central grey 

 matter, especially the nerve-roots, i.e., they are the first to be covered with the myelin. Then 

 fibres which connect the grey matter at different levels are formed the fibres which connect 

 the grey matter of the cord with the cerebellum, and also the former with the tegmentum of the 

 cerebral peduncle. At last the fibres which connect the ganglia of the pedunculus cerebri, and 

 perhaps also the grey matter of the cortex cerebri with the grey matter of the cord are formed. 

 In cases of anencephalous foetuses, i.e., where the cerebrum is absent, neither the pyramidal 

 tracts nor the pyramids are developed. In the braiii before birth, medullated nerve-fibres are 

 formed in the paracentral, central, and oceipital convolutions, and in the island of Reil, and 

 last of all in the frontal convolutions (Ticczek). 



360. SPINAL REFLEXES. By the term reflex movement is meant a 

 movement caused by the stimulation of an afferent (sensory) nerve. The stimulus, 



