SINGLE VISION. 21 



of partition. Thus, for instance, in respect to 

 the eyes, the two optic nerves penetrate their 

 outward coat at the same distance from this 

 plane. Their muscles, blood-vessels, and every 

 other of their component parts and appendages, 

 are arranged in the like manner ; those nearest 

 to the dividing plane, or the innermost, in the 

 one, being similar in structure to the innermost 

 in the other, the outermost to the outermost, 

 and the intermediate to the intermediate. It 

 is surely, therefore, natural to expect, that such 

 parts should also be similar in their properties ; 

 and we in fact find this similarity to exist, where- 

 ever it can be clearly ascertained what the pro- 

 perties are. Every person, for example, admits, 

 that the internal straight muscle of the right 

 eye performs the same office, with respect to 

 that eye, as the other internal straight muscle 

 does with respect to the left eye. What judg- 

 ment are we then to form of the opinion of Dr. 

 Reid, which attributes the same original pro- 

 perties, or rather the joint possession of one ori- 

 ginal property, to places in the retinas situated 

 at unequal distances from the general plane of 

 partition ; which makes an external point in one 

 to correspond, in use, with an internal point in 

 the other, and this too by a principle implanted 

 by nature ? If such things exist, they may,, at 



