120 PHYSIOLOGY FOR DENTAL STUDENTS. 



or 'common salt. This we must add to our food. The extent to 

 which the addition of common salt is made varies very strikingly 

 according to the nature of the organic food taken. When this 

 is mainly vegetable in origin, much common salt is required, the 

 reason being apparently that vegetables contain large quantities 

 of potassium salts which would be harmful unless a proper pro- 

 portion of sodium is also taken. The demand for sodium by 

 herbivorous animals often inclines these to wander for hundreds 

 of miles from their feeding grounds to salt licks. Here they take 

 enough sodium chloride to last them for some time. The carniv- 

 orous animals do not visit salt licks unless it be for the purpose 

 of preying on the herbivorous visitors. The salt hunger from 

 which they suffer compels the the herbivora to the salt licks 

 even in face of this danger of destruction by the carnivora. The 

 same relationship between the desire for salt and the diet is 

 seen in man, for the salt consumption per capita is much greater 

 in rural communities than in those living in towns. 



Usually enough iron is taken either in meats or in certain vege- 

 tables, as spinach. The body is very careful of its supply of 

 iron (which is the most important constituent of haemoglobin), 

 but if it loses it more quickly than the loss can be made good 

 from the food, anemia results and it becomes necessary to pre- 

 scribe iron salts as medicine. 



Similarly with calcium, there is usually enough in the food 

 even of growing animals to meet the demands which bone and 

 teeth formation entails. Rickets is not usually due to a defi- 

 ciency of calcium in'the food, but to a depraved condition of the 

 general nutrition, making it impossible for the available calcium 

 to be properly used. Good food, air and exercise, rather than 

 drugs, is the correct treatment for rickets. 



Our knowledge of just what each particular inorganic salt docs 

 in the metabolism of an animal is not yet very far developed, but 

 some most important discoveries have been made in this connec- 

 tion during recent years. Thus, by observing the isolated beat- 

 ing heart of the frog or turtle it has been found that a certain 

 proportion of sodium, calcium and potassium salts is essential 

 to the maintenance of a proper beat. With sodium chloride 



