and the union of the Optic Nerves. 147 



stated, will be admitted as evidence that such structure does 

 not exist in man. 



A person might probably subject himself to be accused of 

 adopting an indefinite idea, instead of adducing a demonstrable 

 explanation of the interesting phenomenon under examination, 

 were he to say, that the consent of action of the retinas and 

 optic nerves, whereby we see objects single with two eyes, is 

 dependent on sympathy. Nevertheless, the anatomy and. phy- 

 siology of the human body affords several examples, where in 

 like manner we find sympathy, or coincidence of action even 

 of distant parts, to arise from the structure, and depend on 

 juxtaposition of nervous fibres at their origin. This is nowhere 

 more beautifully shown than in the associated action of the in- 

 ternal, with several of the external or accessory muscles of re- 

 spiration. The latter being chiefly supplied by the spinal ac- 

 cessory nerve of Willis, which, though it passes out of the skull 

 with the eighth pair, does arise within the spinal canal, from 

 those pairs of cervical nerves, which externally give off the 

 phrenic nerve to supply the diaphragm. 



The sentiments of Sir Isaac Newton, on the subject of 

 single vision, entirely accord, with the reasoning of Dr Wollas- 

 ton ; indeed, on some points there is almost a verbal accord- 

 ance between these two authors. The following extract and 

 annexed diagram, from Harris's posthumous treatise on optics, 

 published in 1775, proves that the genuine spirit of philoso- 

 phical analysis is the same in all ages ; and shows how the in- 

 ductive reasoning of great minds on the same points, leads them 

 to similar conclusions. 



" And so there are a vast multitude of these slender pipes, 

 which flow from the brain, one-half through the right side nerve 

 I L, Plate II. Fig. 14, till they come to the juncture G F, where 

 they are each divided into two branches, the one passing by G T 

 to the right side of the right eye a b, the other half shooting 

 through the space E F, andpassing by X to the right side of the 

 left eye ef. And in like manner the other half, shooting through 

 the left side nerve M K, divide themselves at F H, and their 

 branches passing by E V to the right eye, and by H Y to the 

 left, compose that half of the retina in both eyes, which is to- 

 wards the left side c d and g h. 



