In the lowest class of animals (e. g., amoeba), we find no special organs for the purpose of respiration, the gas interchange being sufficiently provided for by the exposure of the general surface of their bodies to the medium in which they live, namely, water.

All higher animals have some special apparatus for the purpose of respiration. This apparatus has always the same essential object, that of exposing their tissues to a medium containing oxygen, and of removing the carbonic acid gas.

* The composition of the atmosphere is everywhere remarkably constant, in spite of its oxygen being used up by living beings. It consists of -

Oxygen,.................21 vols, per cent.

Nitrogen,................79 "

Carbonic acid gas (variable),....................................04 "

Moisture (variable),...........8 " ".

In some of the invertebrate animals it suffices to distribute the medium containing oxygen throughout the tissues of the animal by means of tubes. Thus in the Echinodermata a water vascular system exists which seems to carry on the function of respiration. A somewhat similar method of distribution of oxygen takes place in arthropoda, in which delicately branching open tubes (tracheae) distribute air to the tissues of the animal's body.

When more active changes occur in the tissues there is always a perfect blood vascular system. The blood is invariably used as the distributing and collecting agent of the gases in the tissues, and by flowing through some special organ exposed to the surrounding medium it ensures the gas interchange between the body and the outer world. These organs are formed on two general types: (1) external vascular fringes and (2) internal vascular sacs.

Animals living in water have commonly the external fringe arrangement (gills), while those living in air have sacs (lungs). Some animals (frogs, toads, etc.) have gills in the early stages of their life, and lungs when they are more fully developed. In frogs and serpents the lungs are simple sacs, with the inner surface increased by folds of the lining membrane, which gives it a honeycomb appearance; into each sac opens one of the divisions of the air tube. In crocodiles the air tubes divide into several branches, which open into a series of anfractuous, vascular recesses communicating one with another.

Diagram of the Respiratory Organs.

Fig. 144. Diagram of the Respiratory Organs. The windpipe leading down from the larynx is seen to branch into two large bronchi, which subdivide after they enter their respective lungs.

In birds wide bronchial tubes pass through the lung tissue to reach large air cavities. The walls of the tubes are studded with the openings of innumerable air cells lined with capillary blood vessels. The terminal air cavities are not vascular as in the mammalian lung.