676 NERVOUS SYSTEM. 



Transmission of Motcr Stimulus in the Cord. The antero-lateral columns of the cord, 

 in both the white and the gray substance, are entirely insensible to direct irritation, and 

 they conduct the motor stimulus from the centres to the periphery. This statement may 

 be accepted, as the result of positive demonstration, with very little qualification. If the 

 posterior columns of the cord be divided or even removed for a certain length, the animal 

 retains the power of voluntary motion intact. On the other hand, if the antero-lateral 

 columns of the cord be divided on both sides, the power of voluntary motion is lost abso- 

 lutely in all parts supplied with nerves coming from the cord below the section. It 

 would be an interesting point to determine positively the relative importance of the white 

 and the gray substance of the anterior columns in the transmission of motor stimulus ; 

 but this has thus far been impossible. We cannot with certainty divide the gray matter 

 of the anterior columns completely and leave the white substance intact, nor can we 

 divide the white substance without injuring the gray. As far as experiments go, however, 

 they seem to show that transmission is not effected exclusively by the white substance, 

 but that the gray matter plays an important part in this function. We shall refer far- 

 ther on to the action of the gray substance in the transmission of sensory impressions. 



It is evident, from anatomical facts as well as from the results of direct experimenta- 

 tion, that the fibres of conduction of the motor stimulus pass from the brain to the anterior 

 roots of the nerves, through the spinal cord, from above downward, and that there is no 

 other medium for the transmission of the will to the muscles. Wherever the cord be 

 divided, all the muscles supplied by nerves given off below the section are paralyzed. 

 From the brachial enlargement of the cord, nerves of motion pass to the superior extremi- 

 ties, and the inferior extremities are supplied mainly by nerves coming from the lumbar 

 enlargement. The direction of these motor fibres in the cord itself has been elucidated 

 only by experiments upon living animals. If the anterior columns alone be divided in 

 the dorsal region, there is almost complete paralysis of the lower extremities. If the 

 lateral columns be divided in this situation, without injuring the anterior columns, volun- 

 tary movements of the lower extremities are diminished but are not abolished. If the 

 anterior columns be divided high up in the cervical region, there is a diminution in the 

 voluntary movements, but this is by no means so marked as when the section is made in 

 the dorsal region ; but, if the lateral columns be divided in the upper cervical region, the 

 paralysis is almost or quite complete. These facts clearly show that the situation of the 

 chief motor conductors of the cord is different in the dorsal and in the cervical region. 

 In the dorsal region, while conduction of the motor stimulus takes place through fibres 

 contained both in the anterior and in the lateral columns, the transmission is mainly 

 through the anterior columns, the lateral columns being much less important. In the 

 cervical region, the conditions are reversed, and the conduction takes place chiefly by 

 means of the lateral columns. Passing from above downward, therefore, the motor 

 fibres are situated, in the cervical region, mainly in the lateral columns; but progres- 

 sively, as they pass through the dorsal and the lumbar portions of the cord, these fibres 

 change their location and are found chiefly in the anterior columns. 



Recent observations have not sustained the old idea that the lateral columns of the 

 cord contain fibres which preside specially over the movements of the thorax. The 

 experiments of Vulpian upon this point are conclusive. If the lateral column be divided 

 upon one side at about the third or fourth cervical vertebra, there is considerable enfee- 

 blement of the muscles of the thorax upon the corresponding side, but there is also partial 

 loss of power in the limbs, which is more marked in the anterior extremity. This 

 diminution in power in the thoracic muscles is such that, in ordinary tranquil respiration, 

 the side corresponding to the section does not move ; but, in difficult respiration or in 

 crying, the movements are very marked. 



Decussation of the Motor Conductors of the Cord. Well-established anatomical and 

 pathological facts show conclusively that there is a complete decussation of the motor 



