By means of the digestive organs, the food is reduced to a pultaceous mass, and mixed with a variety of secreted fluids. In this state it is denominated Chyme; and, whether it owes its origin to vegetable or animal substances, exhibits, in its last condition, a chemical constitution nearly approaching that of blood, into which it is destined to be converted. In this stage of the process, however, it is necessary to effect a separation between that portion of the chyme which is fitted for the use of the system, and the other part, which is either superfluous or useless.

In giving an account of the circulating system, it is necessary that we attend to the state of the blood, and the fluids which enter into its formation, previous to its aeration ; that we investigate the changes which it undergoes during this important process; and, lastly, consider the uses to which it is applied in what is considered its most perfect state.

The vessels in which the blood is collected previous to aeration, are with great propriety denominated Pulmoinc, by Dr Barclay. They have their origin in every part of the body, and terminate in the lungs, or other organs, in which respiration is performed.

The vessels which are destined to absorb the nutritious portion of the chyme, in the higher orders of animals, are termed Lacteals, from the milky appearance of the fluid which they contain, and which is denominated Chyle.