MINERAL INGREDIENTS OF ANIMAL FOOD. 161 



of food, more phosphorus is taken into the body than it 

 requires ; and the excess has to be carried out in the excre- 

 tions. Sulphur is derived alike from vegetable and animal 

 substances. It exists in flesh, eggs, and milk ; also in the 

 azotized compounds of plants ; and (in the form of sulphate 

 of limp.) in most of the river and spring water that we drink. 

 Iron is found in the yolk of egg, and in milk, as well as in 

 animal flesh ; it also exists, in small quantities, in most 

 vegetable substances used as food by man, such as potatoes, 

 cabbage, peas, cucumbers, mustard, &c. ; and probably in 

 most articles from which other animals derive their support. 



168. Lime is one of the most universally diffused of all 

 mineral bodies ; there being very few animal or vegetable 

 substances in which it does not exist. It is most commonly 

 taken in, among the higher animals, combined with phos- 

 phoric acid, so as to form bone-earth, in which state it exists 

 largely in the seeds of most grasses. A considerable quantity 

 of lime exists, moreover, in the state of carbonate and sul- 

 phate, in all hard water. 



169. When an unusual demand exists for lime, however, 

 for a particular purpose, an increased supply must be afforded. 

 Thus a hen preparing to lay, is impelled by her instinct to 

 eat chalk, mortar, or some other substance containing the car- 

 bonate of lime which is required for the consolidation of the 

 shell ; and if this be withheld, the egg is soft, its covering 

 being composed of animal matter alone, not consolidated by 

 the deposit of earthy particles. The thickness of the shells 

 of aquatic Mollusks depends greatly upon the quantity of 

 lime in the surrounding water. Those which inhabit the sea, 

 find in its waters as much as they require ; but those that 

 dwell in fresh- water lakes, which contain but a small quan- 

 tity of lime, form very thin shells ; whilst, on the other hand, 

 those that inhabit lakes in which, from peculiar local causes, 

 the water is loaded with calcareous matter, form shells of 

 remarkable thickness. 



170. The mode in which the Crustacea, whose calcareous 

 shell is periodically thrown off ( 99), are able to renew it 

 with rapidity, is very curious. There is laid up in the walls 

 of their stomachs a considerable supply of calcareous matter, 

 in little concretions, which are commonly known as " crabs' 

 eyes." When the shell is cast, this matter is taken up by 



