Now dead animal substance and the excreta of animals decompose in the long run into carbonic acid, water and mineral salts, and so there is a continual destruction of animal substance both on the land and in the sea.
The animal excreta, musk and civet, also enter into the composition of modern European pastils and clous fumants.
Peruvian guano is obtained from the excreta of South American sea-birds, and fish guano from the waste of fish.
The washing seems to remove excreta of the muscle's own production, and the period of repose removes them perhaps by diffusion, perhaps by breaking them down into innocuous material.
It has been suggested that the gradual cumulative result of the activity of the nerve cells during the waking day is to load the brain tissue with "fatigue-substances" Theories of which clog the action of the cells, and thus periodi cally produce that loss of consciousness, &c., which is sleep. Such a drugging of tissue by its own excreta is known in muscular fatigue, but the fact that the depth of sleep progressively increases for an hour and more after its onset prevents complete explanation of sleep on similar lines.
The excreta of urea alone thus afford to the soil enormous stores of nitrogen combined in a form which can be rendered available by bacteria, and there are in addition the supplies brought down in rain from the atmosphere, and those due to other living debris.
The exciting cause of sleep is therefore no complete exhaustion of the available material of the cells, nor is it entirely any paralysing of them by their excreta.
The water which bears the oxygen for respiration and the minute organisms upon which the Brachiopod feeds is swept into the mantle cavity by the action of the cilia which cover the arms, and the eggs and excreta pass out into the same cavity.