All animals, from the lowest to the highest, evidence their life by motion at one time or another; and the energy required for this motion is maintained by processes of combustion.

The materials for this combustion, viz. oxygen, and fuel of some sort, or food, are derived from the external medium in which the animal lives; and in order to enable these substances to be available for each part of the animal body, we must have some kind of respiration and circulation going on in it.

In unicellular organisms, consisting of a single mass of protoplasm, the oxygen is derived from the water in which they swim, and both it and the nutritive material derived from the digestion of enclosed masses are circulated through the protoplasm by contractile vacuoles.

In sponges, where the organism no longer consists of one but of several cells united into a community, some of these are furnished with cilia, in order to send a current containing oxygen and food to the other cells having a less favoured position.

In higher animals, where many cells are built up to form one organism, we find a circulatory and respiratory apparatus fully developed.

The medium in which unicellular organisms live is the water in which they swim. The medium in which the cells composing the main parts of the bodies of higher animals, such as man, live, is not the air which surrounds the body, but the intercellular fluid in which the cells themselves are bathed.

As Claude Bernard points out with his usual clearness, the cells of the human body and the lowest unicellular organisms alike live in a liquid medium. From the layer of fluid surrounding it, the cell takes up the oxygen and food which this layer can yield. The supply being exhausted, a unicellular organism can move on elsewhere, but the cells in higher animals, being fixed and unable to move, require fresh portions of oxygen and of nutritive fluid to be brought to them.

This is effected by the slow circulation of the lymph in which the cells themselves are bathed and by the supply to the lymph of oxygen and nutritive material from the blood.

The circulation of the lymph is aided in many lower organisms by the motion of cilia, and this is found persisting in some parts of the higher animals, e.g. the central canal of the spinal cord.

Between the blood and the lymph an interchange goes on, oxygen passing from the blood to the lymph or intercellular fluid, and carbonic acid from the lymph to the blood.

This interchange of gases between the blood, the intercellular fluid, and the cells is termed internal respiration.

In order to maintain this, a constant current of blood must take place; and when its circulation is locally arrested it becomes deprived of oxygen and loaded with carbonic acid, so that the cells in the district in which the stagnation occurs suffer from local asphyxia, while the other parts of the body may be perfectly healthy.

When the general circulation is arrested by stoppage of the heart, by obstruction of the pulmonary arteries, or by the rupture of an aneurism draining the blood away, the whole body suffers in a similar manner from general asphyxia by the cessation of internal respiration.

If oxygen were simply dissolved in the blood, the quantity which would be conveyed to the tissues would be too small for their wants, and we therefore have as an oxygen-carrier a substance capable of taking up a large quantity of oxygen, of readily forming a loose compound with it, and of again giving it off readily to oxidisable substances.

In man and mammals and many of the lower animals this substance is haemoglobin containing iron. In some annelids it is a green substance, chlorocruorin; and in the octopus and some crustaceans it is a blue body, haemocyanin, containing copper.1

In order to remove carbonic acid taken up from the tissues and obtain a fresh supply of oxygen, an interchange takes place between the blood and the external air in the lungs; this is external respiration. Without any direct influence being exerted upon the cells of the animal body themselves, they may be affected and their nutrition greatly modified by:

1st. Alterations in the circulation of the intercellular fluid or lymph in which they are bathed.

2nd. In the greater or less rapidity of circulation of blood locally.

3rd. In the circulation generally, from changes in the heart and blood-vessels generally.

4th. Changes in the oxygen-carrying power of the blood, either from alterations in its power to take up or give off oxygen.

5th. Changes in the external respiration.

All these conditions may be altered by drugs, or at least by therapeutic measures. Thus the circulation of lymph in a part may be increased by shampooing, and its accumulation in a case of dropsy may be removed by incision, by puncture, or by drainage.

The circulation of blood may be arrested locally and gangrene induced by the continuous use of ergot. It may be increased by the use of local stimulants or irritants.

The circulation generally may be affected by the large class of vascular stimulants and depressants, to be afterwards discussed, and sometimes by stoppage of the pulmonary circulation through minute emboli.

Alterations in the oxygen-carrying power of the blood will be discussed presently, and those in the external respiration subsequently.