THE NERVOUS SYSTEM. 99 



over to the other side almost immediately after entering the cord, and 

 along it are transmitted to the brain. There is thus, in the cord itself, 

 an almost complete decussation of sensory impressions brought to it; so 

 that division or disease of one posterior half of the cord (3, Fig. 320) is 

 followed by loss of sensation, not in parts on the corresponding, but in 

 those of the opposite side of the body. From the same fact it happens 

 that a longitudinal antero-posterior section of the cord, along its whole 

 length, most completely abolishes sensibility on both sides of the body. 



c. The various sensations of touch, pain, temperature, and muscular 

 contraction, are probably conducted along separate and distinct sets of 

 fibres. All, however, with the exception of the last named, undergo decus- 

 sation in the spinal cord. 



d. The posterior columns of the cord appear to have a great share in 

 reflex movements. 



e. Impulses of the will, leading to voluntary contractions of muscles, 

 appear to be transmitted principally along the antero -lateral columns; 

 but if a transverse section of this part be made (the grey matter being in- 

 tact) although at first no voluntary movements of the part below occur, 

 this paralysis is only temporary, indicating that the grey matter may take 

 on the conduction of these impulses. 



/. Decussation of motor impulses occurs, not in the spinal cord, as is 

 the case with sensory impressions, but at the anterior part of the medulla 

 oblongata (Fig. 321). Hence, motor impulses, having made their decus- 

 sation, first enter the cord by the lateral tracts and adjoining grey matter, 

 and then pass to the anterior columns and to the grey matter associated 

 with them. Accordingly, division of the anterior pyramids, at the point 

 of decussation (2, Fig. 320), is followed by paralysis of motion in all parts 

 below; while division of the olivary bodies which constitute the true con- 

 tinuations of the anterior columns of the cord, appears to produce very 

 little paralysis. Disease or division of any part of the cerebro-spinal axis 

 above the seat of decussation (1, Fig. 320) is followed, as well-known, 

 by impaired or lost power of motion on the opposite side of the body; 

 while a like injury inflicted below this part (3, Fig. 320), induces similar 

 paralysis on the corresponding side. 



When one half of the spinal cord is cut through, complete anaesthesia 

 of the other side of the body below the point of section results, but there 

 is often greatly increased sensibility (hyperaesthesia) on the same side; so 

 much so that the least touch appears to be agonizing. This condition 

 may persist for several days. Similar effects may, in man, be the result 

 of injury. Thus, in a patient who had sustained a severe lesion of the 

 spinal cord in the cervical region, causing extensive paralysis and loss of 

 sensation in the lower half of the body, there were two circumscribed 

 areas, one on each arm, symmetrically placed, in which the gentlest touch 

 caused extreme pain. 



