Double and Inverted Images on the Retince. 489 



left when doing so, only by their right optic lobes, so they feel 

 and move, on their left, also by the right hemispheres of their 

 brains. And the sensation and motion of the left sides of their 

 bodies are put in connection with the right sides of their brains, 

 only because the laws of light necessarily imply, that the images of 

 whatever lies to their left in the external world, should be formed 

 on the right side of the retinae of their eyes, and impress their 

 right optic lobes. 



If this be the true use of the decussation at the pyramids, the 

 following consequences appear naturally to follow : — 



1. That this piece of structure will be found only in those animals 

 which are, more or less, in the habit of directing both eyes to the 

 same object. 



2. That, where it is found, there will be found also the par- 

 tial decussation of the optic nerves, connecting each eye with 

 both optic lobes, and each optic lobe with both eyes. 



3. That where the decussation at the pyramids exists, — as the 

 sensation and motion of each side of the body will be dependent 

 on the opposite side of the brain, — injuries of either side of the 

 brain, or in general of the parts superior to the decussation, if they 

 produce palsy, will produce it on the opposite side of the body. 



4. That where the decussation at the pyramids does not exist, 

 this crossing of the effect of an injury of the brain to the opposite 

 side of the body will not be observed. 



Now, all these things are so ; at least, according to the most 

 general statements of accurate observers, certainly unconnected 

 with the theory which is here advanced ; and it seems to me ex- 

 tremely improbable that these coincidences should have ex- 

 isted, if the piece of structure in question had not been design- 

 ed for the purpose, and had the effects now stated. 



