ELEMENTS CONTAINED IN PHYSIOLOGICAL COMPOUNDS H 



They show, in the first instance, how the substances with which 

 we have to deal fall under the two main categories of organic and 

 inorganic. In some of the tissues of the body, like bone and tooth, 

 the inorganic or mineral material is in excess, but in the softer 

 portions of the organism the organic compounds are in great pre- 

 ponderance. 



Organic chemistry is sometimes defined as the chemistry of the 

 carbon compounds ; carbon is in all cases present, and is usually the 

 most abundant element. 



The most important of the nitrogenous substances are the 

 proteins, as already explained in the introductory chapter, and the 

 detection and estimation of nitrogen are thus exercises of the highest 

 interest. 



All the proteins contain a small amount of sulphur ; keratin, or 

 horny material, contains more than most of them do. 



Phosphorus is another element of considerable importance, being 

 present in nuclein and nucleo-proteins, and also in certain complex 

 fats, of which lecithin may be taken as a type. Iodine occurs united 

 to protein material in the colloid substance of the thyroid gland ; iron 

 in the pigment of the blood called haemoglobin ; sodium, calcium, 

 potassium, and other metals in the inorganic substances of the body. 

 It would, however, lead us too far into the regions of pure chemistry 

 to undertake exercises for the detection of these and other elements 

 which might be mentioned, and have been already commented upon. 

 The teacher of physiological chemistry is bound to assume that the 

 students who come before him have already passed through a course 

 of ordinary chemistry. 



The main interest of the exercises selected as types lies in their 

 physiological application. As a rule an element is detected by 

 breaking up or oxidising the more or less complex molecule in which 

 it occurs into substances of simpler nature, and then performing tests 

 for these simpler products. Thus carbon is identified by the forma- 

 tion of carbon dioxide, nitrogen by the formation of ammonia, and so 

 forth. 



A great many reactions which can be performed in the test-tube 

 imitate those which are performed in the body. Eeactions in vitro 

 and in vivo, to use the technical phrases, often, though not always, 

 run parallel. Life, from one point of view, is a process of com- 

 bustion or oxidation ; the fuel is supplied by the food ; this becomes 

 assimilated, and so forms an integral part of the living substance of 

 the body ; it is then burnt up by the oxygen brought to it by the 

 blood-stream, giving rise to animal heat and other manifestations of 



