THE CENTRAL NERVOUS SYSTEM 687 



the number of cells in the anterior horn, and of the number of 

 fibres in the anterior roots, it is necessary to assume that one pyra- 

 midal fibre may be connected with several cells. While there is no 

 doubt that the anterior root-fibres and the pyramidal fibres of the 

 brain and cord form segments of the same nervous path, it is still 

 a question whether another neuron may not be intercalated between 

 the pyramidal fibres and the cells of the anterior horn. Schafer 

 supposes that the pyramidal fibres end around some of the cells of 

 Clarke's column, which in turn send off axons that come into rela- 

 tion with the anterior horn cells. 



Connections of the Antero-lateral Descending Tract. These are 

 not well known. Some writers have stated that degeneration is 

 caused in this tract by extirpation of the corresponding half of the 

 cerebellum. This would, of course, indicate that the cells of origin 

 lie in the cerebellum. But recent observations, on the whole, cast 

 doubt upon the statement. Russell, e.g., finds only a few degene- 

 rated fibres in the cord after hemi-extirpation of the cerebellum, 

 although the restiform body contains many degenerated fibres going 

 to the olives. He believes that the main origin of the antero-lateral 

 descending fibres is the nucleus of Deiters, a collection of large 

 multipolar nerve-cells in the floor of the fourth ventricle near the 

 inner auditory nucleus. The fibres appear, pass into the anterior 

 horn, where most of them end by arborizing amongst the cells of 

 the horn. 



Thus far, then, we have been able to map out two great 

 paths from the cerebral cortex to the periphery, one efferent, 

 the other afferent. 



(i) The great efferent or motor pyramidal path, which, starting 

 in the cortex around the fissure of Rolando, where its axons 

 give off numerous collaterals soon after emerging from the 

 cells, and sweeping down the broad fan of the corona 

 radiata, passes through the narrow isthmus of the internal 

 capsule into the crusta of the crus cerebri, and thence into 

 the pons. At this level, the fibres destined to make connec- 

 tion with the motor nuclei of the cranial nerves in the grey 

 matter underlying the aqueduct of Sylvius and the fourth 

 ventricle terminate. Most of these fibres decussate to end in 

 nuclei on the opposite side, but some join nuclei on the 

 same side. On their way through the pons they send off 

 collaterals to the nuclei pontis. The rest of the pyramidal 

 fibres run on into the pyramid of the bulb, where the greater 

 part (usually about 90 per cent.) of the fibres decussate, 

 appearing in the cervical cord as the massive crossed 

 pyramidal tract of the opposite side. A few (usually about 



