And therefore the brain, especially the spinal marrow, which is but the brain prolonged, hath a fairer plea hereto; for these are the principles of motion, wherein dextrality consists; and are divided within and without the Crany. By which division transmitting Nerves respectively unto either side; according to the indifferency, or original and native prepotency, there ariseth an equality in both, or prevalency on either side. And so may it be made out, what many may wonder at, why some most actively use the contrary Arm and Leg; for the vigour of the one dependeth upon the upper part of the spine, but the other upon the lower. And therefore many things are Philosophically delivered concerning right and left, which admit of some suspension. That a Woman upon a masculine conception advanceth her right Leg, will not be found to answer strick observation. That males are conceived in the right side of the womb, females in the left, though generally delivered, and supported by ancient testimony, will make no infallible account; it happening oft times that males and females do lie upon both sides, and Hermaphrodites for ought we know on either. It is also suspitious what is delivered concerning the right and left testicle, that males are begotten from the one, and females from the other. For though the left seminal vein proceedeth from the emulgent, and is therefore conceived to carry down a serous and feminine matter; yet the seminal Arteries which send forth the active materials, are both derived from the great Artery. Beside this original of the left vein was thus contrived, to avoid the pulsation of the great Artery, over which it must have passed to attain unto the testicle. Nor can we easily infer such different effects from the divers situation of parts which have one end and office; for in the kidneys which have one office, the right is seated lower then the left, whereby it lieth free, and giveth way unto the Liver. And therefore also that way which is delivered for masculine generation, to make a strait ligature about the left testicle, thereby to intercept the evacuation of that part, deserveth consideration. For one sufficeth unto generation, as hath been observed in semicastration, and oft times in carnous ruptures. [SN: _How an Horse or Bull may generate after they be gelt._] Beside, the seminal ejaculation proceeds not immediately from the testicle, but from the spermatick glandules; and therefore _Aristotle_ affirms (and reason cannot deny) that although there be nothing diffused from the testicles, an _Horse_ or _Bull_ may generate after castration; that is, from the stock and remainder of seminal matter, already prepared and stored up in the Prostates or grandules of generation.