PHILOSOPHICAL TRANSACTIONS. 541 



or superior fibre in the other thalamus, which run in a like manner to the su- 

 perior part of each eye, I call fibrae Concordes, as aa^ bb, cc, &c. and answer 

 to unisons in 2 viols or lutes, and therefore may be also called fibrae homotonae. 

 These arise separately from the brain ; nor was there any necessity of their 

 union at their first origin, in order to their consent, since they answer so ex- 

 actly in their site and tension. In like manner are those correspondent, that 

 are in the lowermost part of the thalami optici, and run to the lowest part of 

 the eye, because they have the least flexure or tension ; and therefore the in- 

 termediate fibres have consequently the greater tension proportionably as they 

 are nearer the top, or less, as they are nearer the basis, only with this dif- 

 ference, that the internal lateral fibres have less tension than the external, as 

 before-mentioned. Hence it appears that the fibres that are parallel, or in the 

 same position, are as it were symphonical, viz. the 2 superiors, 2 inferiors, ex- 

 ternal-lateral, and internal-lateral, in each thalamus opticus, exactly answer 

 each other in site and tension ; so that when any impression from an external 

 object moves both fibres, it causes not a double sensation, any more than uni- 

 sons in 2 viols struck together cause a double sound. Now this parallelism of 

 the fibres is much secured by what we are next to consider, viz. 



The position of the fibres of the optic nerve, which keep their distinct or- 

 der. For that the nerves decussate or cross one another, or that they are so 

 blended together in their union, as to cause a confusion in their fibres, is not 

 to be imagined; but those that are in the thalami optici on the right side run 

 distinctly to the right eye, and those on the left accordingly. In many fishes 

 the case is clear, where the 2 nerves are joined only by simple contact, and in 

 the camelion not at all, as is said. 



Besides this union of the nerves, to keep the balance, it is also much as- 

 sisted by the excellent contrivance of the muscles. For it is very observable, 

 that the musculus obliquus inferior arises out of a peculiar foramen of the or- 

 bit, whereas all the rest arise from the bottom at the exit of the optic nerve; 

 so that, both below by this muscle, and. above by the trochlearis, a distortion 

 of the eye is prevented. And as the two oblique muscles are equally poised, so 

 are the attollens and deprimens ; for though the former be somewhat the larger 

 muscle, yet because there is less force required to pull down than lift up the 

 eye, their powers are equal; as for the lateral muscles, they answer exactly both 

 in site and size, and consequently keep the balance, which is required to the 

 consent of the fibrae Concordes. 



The insertion of the fibres of the optic nerve into the bottom of the eye, 

 and their keeping still their distinct position^ according as they arise from the 



