Proteids. The proteids or albuminoids are an important class of organic compounds. In very many of their properties they closely resemble one another, while their chemical composition, or at least their percentage chemical composition, only varies within slight limits. They all contain hydrogen, carbon, oxygen, and nitrogen, while small quantities of sulphur (and often phosphorus) are usually present. Their importance to the human economy is due to the fact that animals are incapable of deriving their supply of nitrogen necessary for the maintenance of life from inorganic sources of this element. Proteids are hence an essential food-stuff, deprivation of which results in starvation. The proteids in the system pass into the blood only after their conversion into a soluble proteid - peptone - which is effected by the fermentation induced by the pepsin of the gastric juice and the pancreatin of the pancreatic secretion, both of which substances are themselves proteid materials. Amongst the more common of these compounds are: albumen, present in white of egg and in blood serum; myosin, the proteid material of muscle and lean meat; casein, existing in milk and cheese; fibrin, present in the blood; globulin, a component of many animal tissues; chondrine, which exists in cartilage; vitellin, in the yolk of eggs. Gelatine appears to be very closely allied to the proteids, but cannot act as a nitrogenous food in the same manner as the proteids proper. Many substances also of vegetable origin also are closely allied to these animal compounds, and are spoken of as vegetable proteids.