SUBSTANCES WHICH PASS THROUGH THE ORGANISM. 435 



mechanical uses of these parts, giving them their power of resistance without 

 rendering them too brittle. It is more abundant in the bones of the lower 

 extremities, which have to sustain the weight of the body, than in the upper 

 extremities ; and in the ribs, which are elastic rather than resisting, it exists 

 in less quantity than in the bones of the arm. 



The necessity of a proper proportion of calcium phosphate in the bones 

 is made evident by cases of disease. In rachitis, where, as is seen by the 

 table, its quantity is very much diminished, the bones being unable to sus- 

 tain the weight of the body become deformed ; and finally, when calcium 

 phosphate is deposited, they retain their distorted shape. 



Origin and Discharge of Calcium Phosphate. The origin of calcium 

 phosphate is exclusively from the external world. It enters into the consti- 

 tution of food and is discharged in the faeces, urine and other matters thrown 

 off by the body. Its proportion in the urine is very variable. 



Calcium Carbonate. This salt exists in the bones^ teeth, cartilage, internal 

 ear, blood, sebaceous matter and sometimes in the urine. It exists as a nor- 

 mal constituent of the urine in some herbivora but not in the carnivora or 

 in man. It is most appropriately considered immediately after calcium phos- 

 phate, because it is the salt next in importance in the constitution of the 

 bones and teeth. In these structures it exists intimately combined with the 

 organic matter, under the same conditions as the phosphates, and it has analo- 

 gous uses. In the fluids it exists in small quantity and is held in solution 

 by virtue of free carbon dioxide and potassium chloride. 



Calcium carbonate is the only example of an inorganic salt existing un- 

 combined and in a crystalline form in the body. In the internal ear it is 

 found in this form and has some office connected with audition. 



TABLE OF QUANTITIES OF CALCIUM CARBONATE. 



Parts per 1,000. 



In bone, human (Berzelius) 113-00 



" " " (Marchand) 102-00 



" " " (Lassaigne) 76-00 



In teeth of an infant one day old j ( 140-00 



In teeth of an adult > Lassaigne. < 100-00 



In teeth of an old man, eighty-one years . ) ' 10*00 



In urine of the horse (Boussingault) 10*82 



Origin and Discharge of Calcium Carbonate. This salt is introduced into 

 the body with food, held in solution in water by the carbon dioxide, which is 

 always present in small quantity. It is also formed in the body, particularly 

 in the herbivora, by a decomposition of the calcium tartrates, malates, citrates 

 and acetates contained in the food. These salts, meeting with carbon diox- 

 ide, are decomposed and calcium carbonate is formed. It is probable that 

 in the human subject some of it is changed into calcium phosphate and in 

 this form is discharged in the urine ; but it has not been definitely ascertained 

 when and how this change takes place. 



Sodium Carbonate. This salt is found in the blood and saliva, giving to 

 these fluids their alkalinity ; in the urine of the human subject when it is 



