The Animal Kingdom (as described by Linnaeus): Animals enjoy sensation by means of a living organization, animated by a medullary substance; perception by nerves; and motion by the exertion of the will.

They run outward in a straight course between the tubes of the medullary substance, and joining, as above stated, the interlobular veins, form venous arcades; these in turn unite and form veins which pass along the sides of the renal pyramids.

Hartley believed that sensation is the result of vibrations of the minute particles of the medullary substance of the nerves, made possible by a subtle, elastic ether that was rare in the interstices of solid bodies and in their close neighbourhood.