CHAP. XVII. Of the nerues of the Eyes, or of the first and second Coniugations.
ALbeit in a particular discourse hereafter following wee intend more dis∣trictly and carefully to prosecute the Nature and diuarications of nerues in generall and euery one in particular; yet because all the nerues of the head haue their originall from diuers partes of the substance of the same: Wee cannot chuse in this place but make some mention of them though * 1.1 it be the briefer.
The first coniugation therefore of the nerues of the braine are the Opticks, [Table 17. figure 1. and 2. G] larger and softer then all other simple nerues. Larger because they bee sensibly perforated, softer because they arise out of the Center of the Basis, whence procee∣ding forward with a slow course they are vnited aboue the saddle of the wedge-bone [tab. 17. figure 1. H table 14. figure 19. O] and so make a common body. After being againe se∣parated [Table 14. figure 19. M N Table 17. figure 1. aboue H] they passe obliquely foreward through their proper hole] table 4. figure 10. B] the one to the Center of the right eye, the other to the Center of the left [ta. 3. lib. 8. fig. 8. a or together with the membrane fig. 2. abc. * 1.2]
They consist of a double substance, the one marrowey the other membranous. The marrowey is soft like the substance of the braine, which you shall perceiue to yssue if it bee * 1.3 hard pressed, and of this marrowy substance dilated in the orbe of the eye is that coat made which of the similitude of a net is called Retina tunica [Tab. 17. fig. 1 and 2. I] and this com∣passeth the glassy humour like a sphere. * 1.4
It hath two membranes from the two Meninges of the braine. The thin membrane in∣uesteth it from the originall to his out-let from the skul; the thick Meninx is added to it, frō thence to the eye; and so the thicke membrane degenerateth into the horny coate, the thin is consumed or spent in the Vuea, and so the Animall spirit is transported through the con∣tinuation of this nerue vnto the pupilla or Apple, which is the hole of the grapie mēbrane.
They are called therefore Opticke nerues from their action, because they communi∣cate to the eye the sence of seeing.
The second coniugation is of the nerues which moue the eyes, [table 18. figure 1. and 2. * 1.5 • tab. 15. figure 20. G] which is distinguished from the former only by a little and thin bone, and riseth from the Basis of the braine [tab. 25. fig. 1. C] a little behinde the former as Vesali∣us saith. This is smaller and slenderer by much then is the opticke, as also harder because it was to be inserted into the muscles. It yssueth by a proper hole of his owne [tab. 4. fig. 10. G] which is the second of the wedge-bone, where after it is diuided into notable braunches which are sent into the muscles. In some bodies (though it be but seldom) they send small * 1.6 twigs to the temporall muscles; whence it is that sometimes if that muscle be offended the eie also suffereth with it, and so on the contrary.
Their vse is to afford to the muscles the facultie of Motion, as also a drawing power to their membranes. Now the reason why when one eie is mooued to the side the other also * 1.7 necessarily followeth, is, because these nerues are continued one with another in their ori∣ginall.