DUAL CHARACTER OF THE BRAIN. Y 



one only would be enough, which demonstrate that the theory 

 ought to be rejected. 



But as regards sight we find this, and it is a point of impor- 

 tance, that a disease anywhere in one-half of the brain can exist 

 without any alteration of sight whatever. A disease existing 

 in that part where the optic nerve enters the brain, destroying 

 that part altogether, may not be a cause of loss of sight ; so 

 that one optic fibre alone may be perfectly suflScient for the 

 functions of the two eyes. Therefore I conclude that it is quite 

 enough to have one brain to have our power of sight ; and as 

 it is so for each half of the brain, I can also conclude, and 

 this is a point of importance, that each half of the brain is inde- 

 pendent of the other, and that each of them possesses the 

 powers of serving to the sensations of sight. You will ask 

 how is it that a disease in certain cases in the brain will produce 

 loss of sight, and that in other cases a disease in the same 

 part will not produce loss of sight. As regards this, I cannot 

 develop at length what I would have to say; but if any of 3'ou 

 were present at ray lecture in this city last year, or at the 

 Academy of Sciences to-day, you know that an alteration in 

 any part of the nervous system, whether in the brain or else- 

 where, can, by producing an irritation, act on other parts, so 

 as to produce the loss of a function of those other parts ; and 

 so it is especially with sight. In many experiments I have 

 ascertained that injuring a small part of the spinal cord pro- 

 duces a loss of sight in the eye on the same side. An injury to 

 tlie medulla oblongata, a little higher than the part of the spinal 

 cord which produces loss of sight on the same side, will produce 

 a loss of sight, but in the opposite eye. There is, therefore, a 

 power of producing a loss of sight by irritation ; and indeed 

 there is nothing more common in children having worms than 

 a diminution in the power of seeing. It is in the same way that 

 an irritation existing in certain parts of the brain will produce, 

 at a distance from the place where it exists, a loss of the func- 



