78 PROXIMATE PRINCIPLES OF THE FIRST CLASS. 



and their definite chemical composition, but also in the part which 

 they take in the constitution of the animal frame. They are 

 distinguished in this respect, first by being derived entirely from 

 without. There are a few exceptions to this rule ; as, for example, 

 in the case of the alkaline carbonates, which partly originate in 

 the body from the decomposition of malates, tartrates, &c. These, 

 however, are only exceptions ; and in general, the proximate prin- 

 ciples belonging to the first class are introduced with the food, 

 and taken up by the animal tissues in precisely the same form 

 under which they occur in external nature. The carbonate of lime 

 in the bones, the chloride of sodium in the blood and tissues, are 

 the same substances which are met with in the calcareous rocks, 

 and in solution in sea water. They do not suffer any chemical 

 alteration in becoming constituent parts of the animal frame. 



They are equally exempt, as a general rule, from any alteration 

 while they remain in the body, and during their passage through 

 it. The exceptions to this rule are very few ; as, for example, where 

 a small part of the chloride of sodium suffers double decomposition 

 with phosphate of potassa, giving rise to chloride of potassium and 

 phosphate of soda ; or where the phosphate of soda itself gives up 

 a part of its base to an organic acid (uric), and is converted in this 

 way into a bi-phosphate of soda. 



Nearly the whole of these substances, finally, are taken up un- 

 changed from the tissues, and discharged unchanged with the excre- 

 tions. Thus we find the phosphate of lime and the chloride of so- 

 dium, which were taken in with the food, discharged again under 

 the same form in the urine. They do not, therefore, for the most 

 part, participate directly in the chemical changes going on in the 

 body ; but only serve by their presence to enable those changes to 

 be accomplished in the other ingredients of the animal frame, which 

 are necessary to the process of nutrition. 



