Capillaries

‘... big, rounded structures without any apparent internal cavity, slipping from the skull or dorsal vertebrae and carrying down animal spirit from the brain to produce the animal faculty for the parts of the body ... I scarcely dare to deny the hollowness of the nerve although I have never seen a channel even in the optic nerve; this despite the fact that the nerves, like the veins and arteries, are called vessels and that I am aware that professors of anatomy declare the optic nerves to be hollow.’