And I say briefly, there are eight general causes that hinder the passage of the spirit to the nerves. The first cause is poverty and scantiness of spirit and heat, so that they cannot increase and spread themselves to the nerves.578 The second cause is excess of cold, which retains579 and binds together the nerves.
The third is excess of heat, which dissipates the spirit, such as burning fevers,580 and remaining too long in an overhot bath. The fourth cause is dryness,581 that twists the nerves, the which cause is bad. The fifth cause is moisture, without582 matter having come to block it up. The sixth cause is excessive tightening and binding of the member. The seventh cause, a stoppage from whatever cause it come; whether from red blood, because of its quantity, or phlegm because of its sluggishness, or melancholy because of its toughness and denseness, or from choler on account of its thinness, which causes the other humours, with which it is mixed, to proceed so that they penetrate throughout.583 Therefore since melancholy causes stoppage, there is no clear passage through the nerves, except those of the eye584 and the virga, as Galen says. The eighth cause is hardening and thickening of the nerves.