It has already been mentioned that the cells of which higher organisms are composed live in the intercellular fluid or lymph which bathes them.

This nutritive fluid is continually being renewed by fresh supplies exuding from the blood-vessels into the lymph-spaces which surround the cells, the excess being removed by absorption either by the veins or by the lymphatics. Besides this, an interchange of gases (internal respiration) and of solids takes place by diffusion between the lymph and the blood.

When the circulation stops, internal respiration is arrested, and the cells die. But they do not all die at the same time, for some are able to live longer without fresh supplies of oxygen than others. The order in which they die is (1) the cells of the initiative nerve-centres, as the brain; (2) those of the automatic and reflex centres; (3) nerve-fibres (which are modified nerve-cells); (4) unstriated muscles; (5) striated muscles.