The Anatomy of Melancholy

What causes movement is either its efficient cause or its end. The end is the object desired or avoided, as when a dog seeks to catch a hare. The efficient cause in humans is reason, or imagination acting under it, which perceives good or bad objects. In other animals it is imagination alone, which moves appetite; appetite moves this faculty, which, through a wonderful bond established by nature and through the mediation of the spirit, commands the organ by which it acts.

That organ consists of nerves, muscles and tendons distributed throughout the body, contracting and relaxing as the spirits direct. The spirits move the muscles, or the nerves within them, and draw the tendon—and consequently the joint—to the intended position. What is moved is the body or some part capable of movement.

The body’s movements are various: walking, running, leaping, dancing, sitting and the like, referred to the category of bodily position. Worms creep, birds fly and fish swim. There are also movements of the parts, the chief being respiration, or breathing, which takes place as follows.

The outside air is drawn in through the windpipe and sent, through the action of the diaphragm, to the lungs. Expanding like a pair of bellows, they alternately draw it in and send it out to the heart to cool it; then they carry it back from there, now warmed, continually taking in fresh air. The pulse has a similar movement, about which I will say nothing, since many have written whole books on it.