Finally, reference should be made here to several other products of tissue metabolism, products of the breaking down of proteid matter in the body, since they are liable to prove of interest to us in other connections. Thus creatin, abundant in the muscle and other places; the related substance creatinin, present in the urine; methyl guanidin, a decomposition product of creatin; and urea, all call for a word of description. The chemical relationship of these bodies is clearly indicated by the following formulæ: NH_{2} NH------- / / \ C==NH C==NH CO \ \ / N(CH_{3})CH_{2}COOH N(CH_{3})CH_{2} Creatin Creatinin NH_{2} NH_{2} / / C==NH C==O \ \ NH(CH_{3}) NH_{2} Methyl guanidin Urea Creatinin is chemically the anhydride of creatin, _i. e._, it can be formed from creatin by the simple extraction of one molecule of water, H_{2}O. Creatin, by hydrolytic cleavage, will break down into one molecule of urea and one molecule of sarcosin or methyl glycocoll, as shown in the following equation: NH_{2} NH_{2} / CH_{2}·NH·(CH_{3}) / C==NH + H_{2}O = | + C==O \ COOH \ N (CH_{3}) CH_{2}COOH NH_{2} Creatin Sarcosin Urea