718 



THE NERVOUS SYSTEM. 



what atrophied. Under favorable conditions the cell is capable of reassuring its 

 normal appearance, and the axon may commence to grow. This method of inject- 

 ing methylene-blue is of great value in determining the origin of nerve-fibres from 

 their cells. Again, stimulation of certain localized areas of the brain or of the 

 tracts arising from them is followed by the contraction of the muscles of the body. 

 These cortical centres of the motor tracts are situated in the convolutions adjacent 

 to the fissure of Rolando. When the stimulus is applied to one part the muscles 

 of the hind limb contract, while other portions control the movements of the fore 

 limb, etc. Destruction of these parts entails loss of function, paralysis of muscles, 

 and degeneration of the tracts below the seat of injury. During life injury and 

 disease may give rise to symptoms resembling either the eifects of stimulation or 

 those of destruction ; and after death the tracts, or the centres of the tracts, are 

 seen to be degenerated or otherwise altered. Further, by observing the develop- 

 ment of the nervous system during the growth of the embryo, the fact is disclosed 

 that all axis-cylinders do not acquire a medullary sheath at one and the same time. 

 Speaking generally, it may be said that afferent fibres become medullated before 

 efferent, and that in the case of the latter myelination occurs earlier in the brain 

 than in the cord. By watching the effects of these different processes the func- 

 tions of a considerable part of the brain and of the nerves leading from or to it 

 have been determined. 



The Motor, Efferent, or Descending Tract. 



The constituent fibres of this tract are the axis-cylinder processes of cells situated 

 in the cortex of the convolutions around the fissure of Rolando. At first they are 

 somewhat widely diffused, but as they descend through the corona radiata they 



gradually approach each other and 

 pass between the lenticular nucleus 

 and optic thalamus in the genu and 

 anterior two-thirds of the posterior 

 limb of the internal capsule. Pro- 

 ceeding downward they next oc- 

 cupy the middle of the pes or 

 crusta of the crus cerebri, and 

 enter the pons Varolii, where the 

 transverse fibres of this body not 

 only conceal them, but divide them 

 up into irregular bundles. Event- 

 ually they reach the medulla, and 

 here the motor tracts form the 

 anterior pyramids which lie one 

 on each side of the median fissure. 

 The transit of the fibres from the 

 medulla is effected by two paths. 

 The fibres nearest to the anterior 

 median fissure cross the middle 

 line, forming the decussation of the 

 pyramids, and descend in the op- 

 posite side of the cord as the indi- 

 rect or crossed pyramidal tract. 

 Throughout the length of the 

 spinal cord fibres from this column 

 pass into the gray matter, to ter- 

 minate by ramifying around the cells of the anterior horn. The more laterally 

 placed portion of the motor tract does not decussate in the medulla, but descends 

 as the direct or uncrossed pyramidal tract ; these fibres, however, end in the ante- 

 rior gray horn of the opposite side of the spinal cord by passing across in the 



FIG. 388. Dorsal roots entering cord and dividing into 

 ascending and descending branches. (Van Gehnchten.) a, 

 Stem-fibre; 6, 6, ascending and descending limbs of bifurca- 

 tion ; c, collateral arising from stem-fibre. 



