But although protoplasm is so complex chemically as to defy exact analysis, being an elaborate structure of atoms built up into a molecule in which each atom must occupy its true place (like every carved stone in a Gothic cathedral), yet it is, as it were, only the starting-point or material out of which the infinitely varied structures of living bodies are formed. The extreme mobility and changeability of the structure of these molecules enables the protoplasm to be continually modified both in constitution and form, and, by the substitution or addition of other elements, to serve special purposes. Thus when sulphur in small quantities is absorbed and built into the molecular structure, proteids are formed. These are most abundant in animal structures, and give the nourishing properties to meat, cheese, eggs, and other animal foods; but they are also found in the vegetable kingdom, especially in nuts and seeds such as grain, peas, etc. These are generally known as nitrogenous foods, and are very nutritious, but not so easily digestible as meat. Proteids exist in very varied forms and often contain phosphorus as well as sulphur, but their main characteristic is the large proportion of nitrogen they contain, while many other animal and vegetable products, as most roots, tubers, and grains, and even fats and oils, are mainly composed of starch and sugar. In its chemical and physiological aspects protein is thus described by Professor W.D. Haliburton:--'Proteids are produced only in the living laboratory of animals and plants; proteid matter is the all-important material present in protoplasm. This molecule is the most complex that is known; it always contains five and often six or even seven elements. The task of thoroughly understanding its composition is necessarily vast, and advance slow. But, little by little, the puzzle is being solved, and this final conquest of organic chemistry, when it does arrive, will furnish physiologists with new light on many of the dark places of physiological science.'[13]