CROSSED ACTION OF THE SPINAL CORD. 455 



constitutes the so-called "decussation of the anterior pyramids;" and 

 beyond this point the longitudinal fibres of the medulla oblongata con- 

 tinue their course toward the peduncles of the brain and the cerebral 

 ganglia above. Thus the crossing of the tracts for voluntary motion is 

 completed in the lower half of the medulla oblongata. Injury of one 

 lateral half of the brain above this situation causes muscular paralysis 

 on the opposite side of the body ; injury of one lateral half of the spinal 

 cord below it causes paralysis on the same side of the body. 



These are the general results obtained from both pathological observa- 

 tion and physiological experiment; and they evidently point to the 

 medulla oblongata as the principal or exclusive seat of the bilateral 

 decussation of the channels for voluntaiy motion. At the same time it 

 is shown, by mircroscopic examination, that this is not the only spot 

 where an anatomical intercharge of fibres takes place between the two 

 lateral halves of the spinal cord. On the contrary, a decussation of 

 fibres exists everywhere, throughout the length of the cord, from left to 

 right, and vice versa, at the situation of the " white commissure," at the 

 bottom of the anterior median fissure ; the right anterior column con- 

 stantly receiving fibres from the left side of the cord, and the left anterior 

 column from the right side of the cord. This continuous decussation is 

 concealed from view externally, and is only discoverable by means of 

 transverse microscopic sections ; while that at the level of the anterior 

 pyramids is easily visible, owing to the size of the decussating bundles 

 and their oblique direction. 



The anatomical distinction between the two sets of decussating fibres 

 may answer to a corresponding difference in their physiological action ; 

 and the decussation at the white commissure may be connected with 

 the reflex action of the cord, or with the simultaneous action of its tw r o 

 opposite sides. It is certain that this commissure does not take part in 

 the transmission of voluntary impulses; since in the celebrated experi- 

 ment of Galen, 1 "if the spinal cord be divided by a longitudinal section, 

 from above downward, in the median line," so as to separate its two 

 lateral halves from each other, this operation is not followed by loss 

 of motion either on one side or the other. This result has also been 

 obtained by Brown-Se'quard 2 in the lumbar region, voluntary motion 

 being retained in both the posterior limbs. On the other hand, as 

 shown by the same observer, a longitudinal section of the medulla 

 oblongata alone in the median line, so as to divide the decussating fibres 

 of the anterior pyramids, produces complete loss of voluntary movement 

 in all the limbs at once. 



Decussation of the Sensitive Tracts. The sensitive impressions, 

 conveyed from the integument to the nervous centres, undergo, like the 

 motor impulses, a complete bilateral decussation by the time they arrive 



1 De Administrationibus Anatomicis. Liber viii. cap. vi. 



2 Physiology and Pathology of the Central Nervous System. Philadelphia, 

 1860, p. 33. 



