492 Dr Alison on Single and Correct Vision, by means of 



In answer to this, I would observe, first, that although there 

 may be an anatomical difficulty in understanding how the sensa- 

 tion and motion of the left side of the face are put in connection 

 with the right side of the brain, yet observation of the effects of 

 injury or disease demonstrate that they are in connection with 

 it ; the palsy depending on injury or disease of the right side of 

 the brain extending very generally to the left side of the face, as 

 well as of the body. And secondly, I would say that there is no 

 great difficulty in understanding that this should be the case, if 

 we suppose, as Mr Mayo appears inclined to do,* that the true 

 origin of the nerves is in those columns of the cerebro-spinal axis 

 which do not decussate (and this is pretty certainly the case as to 

 the fifth nerve, which is easily traced down into the spinal cord be- 

 hind the Corpora pyramidalia), and that the decussating fibres of 

 the Corpora pyramidalia are not to be considered as the continua- 

 tion of the columns of the spinal cord, but as the cords of commu- 

 nication between these columns, and the masses of the brain and 

 cerebellum, f If this be so, it is easy to conceive, that an injury of 

 the right side of the brain, transmitted through these cords of com- 

 munication, should strike downwards to the left side of the body, 

 and upwards to the left side of the face in palsy ; and that, in the 

 natural state, the sensation and motion of the left side of the face, 

 as well as trunk and limbs, should be in connexion, although by 

 a circuitous route, with the right side of the brain ; and therefore 

 in harmony with impressions on the right optic lobe. 



If the foregoing speculation be just, it would seem that the 

 structure to which the attention of physiologists was first directed 

 by Newton, is only a part of the arrangements, by which the in- 



• Outlines of Human Pathology, p. 202. 



f " The two cords of the spinal marrow do not cross, but merely the middle or 

 pyramidal fasciculi of each, which give origin to the Crura cerebri by expanding and 

 becoming broader." — Tiedemanri 's Anatomy of the Foetal Brain, Translated by Ben- 

 nett, p. 144. I am aware of the difficulty of tracing the course of the fibres at the 

 medulla oblongata, and what Sir Charles Bell describes, I believe correctly, decus- 

 sating fibres behind the pyramids ; but all are agreed that there are fibres in this 

 part which do not decussate. 



