658 FUNCTIONS OF THE SPINAL CORD. [CH. XLVII. 



same side of the cord, and loss of motion and sensation on the 

 same side of the body below the lesion (see p. 616). 



The motor path in the cord from the brain is the pyramidal 

 tract ; the anatomy of this tract is described in Chapters 

 XLII. to XLVL, and we need do no more here than remind the 

 reader that it originates from the pyramidal cells of the cortex of 

 the opposite cerebral hemisphere, and that the principal decussa- 

 tion occurs at the lower part of the bulb. 



The sensory tracts are more complex, on account of the nume- 

 rous cell-stations on their course. The path for tactile and mus- 

 cular sense impressions is up the posterior columns to the'nucleus 

 gracilis and nucleus cuneatus ; thence by the internal arcuate fibres 

 and fillet to the optic thalamus of the opposite side, and thence 

 by the posterior part of the internal capsule to the Rolandic area of 

 the hemisphere ; the decussation of the fillet occurs in the bulb. 



Schiff, one of the earliest to work at the subject of con- 

 ducting paths in the cord, arrived at the conclusion that painful 

 impressions travelled to the brain by the grey matter of the 

 cord. This conclusion was regarded as paradoxical, for white 

 matter is conducting, grey matter is central or reflecting. But 

 the conclusion is not so paradoxical as it appears at first 

 sight, for we now know the grey matter is made up of nerve- 

 units, communicating physiologically by their interlacement of 

 dendrons ; and it is quite easy to understand that impulses may 

 travel up grey matter through a vast series of cell stations or 

 positions of relay. The more exact methods -of modern research 

 have gone far to justify Schiff's conclusions, and it is now gene- 

 rally held that the impulses due to painful impressions, and also 

 those produced by heat and cold, travel up to the optic thalamus 

 by the loopings of fibres from cell to cell through the tract of 

 grey matter, which is continuous from cord to optic thalamus 

 (fig. 484, G.M.) ; from the optic thalamus the fibres of the corona 

 radiata carry on the impulse to the cortex. These conclusions are 

 confirmed by recent experiments on hemisection (see p. 616), and 

 by the phenomena seen in certain diseases. One of the most 

 instructive of these from the physiological standpoint is known 

 as locomotor ataxy. This disease is an affection of the afferent 

 channels, and the most marked and constant change in the spinal 

 cord is a degenerative one in the posterior columns. In such a 

 case muscular and tactile sense are abolished, particularly in the 

 lower limbs, but painful and thermal sensations are felt. On the 

 other hand, in the disease of the grey matter of the cord called 

 syringomyelia, sensations of heat, cold and pain are lost, and 

 tactile sensations remain. 



