In connexion with the heart there are three phenomena, which, though

apparently of the same nature, are really not so, namely

palpitation, pulsation, and respiration.

Palpitation is the rushing together of the hot substance in the

heart owing to the chilling influence of residual or waste products.

It occurs, for example, in the ailment known as 'spasms' and in

other diseases. It occurs also in fear, for when one is afraid the

upper parts become cold, and the hot substance, fleeing away, by its

concentration in the heart produces palpitation. It is crushed into so

small a space that sometimes life is extinguished, and the animals die

of the fright and morbid disturbance.

The beating of the heart, which, as can be seen, goes on

continuously, is similar to the throbbing of an abscess. That,

however, is accompanied by pain, because the change produced in the

blood is unnatural, and it goes on until the matter formed by

concoction is discharged. There is a similarity between this

phenomenon and that of boiling; for boiling is due to the

volatilization of fluid by heat and the expansion consequent on

increase of bulk. But in an abscess, if there is no evaporation

through the walls, the process terminates in suppuration due to the

thickening of the liquid, while in boiling it ends in the escape of

the fluid out of the containing vessel.

In the heart the beating is produced by the heat expanding the

fluid, of which the food furnishes a constant supply. It occurs when

the fluid rises to the outer wall of the heart, and it goes on

continuously; for there is a constant flow of the fluid that goes to

constitute the blood, it being in the heart that the blood receives

its primary elaboration. That this is so we can perceive in the

initial stages of generation, for the heart can be seen to contain

blood before the veins become distinct. This explains why pulsation in

youth exceeds that in older people, for in the young the formation