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 lime) 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 
M 
