160 MINERAL INGREDIENTS OF FOOD. important. It exists largely in the blood, and in the various animal fluids which are secreted from it; and it is also an essential ingredient of most of the solid tissues. Its presence obviously tends to prevent that spontaneous decomposition to which organic substances are liable. Phosphorus is chiefly required to be united with fatty matter, to serve as the material of the nervous tissue; and to be combined with oxygen and lime, to form the bone-earth by which the bones are consolidated. Sulphur exists in small quantities in several animal tissues; but its part seems by no means so important as that performed by phosphorus. Lime is required for the consolidation of the bones, and for the production of the shells and other hard parts that form the skeletons of the Invertebrata. Of the limestone rocks of which a great part of the crust of our globe is composed, a very large proportion is made up of the remains of animals that formerly existed in the ocean. Thus some almost entirely consist of masses of Coral, others of beds of Shells, and others of the coverings of minute Foraminifera (§ 131). To these mineral ingredients we may also add Iron, which is a very important element in the red blood of Vertebrated animals. 167. These substances are contained, more or less abundantly, in most articles generally used as food; and where they are deficient, the animal suffers in consequence, if they be not supplied in any other way. Common Salt exists, in no inconsiderable quantity, in the flesh and fluids of animals, in milk, and in the egg: it is not so abundant, however, in plants; and the deficiency is usually supplied to herbivorous animals by some other means. Thus salt is purposely mingled with the food of domesticated animals; and in most parts of the world inhabited by wild cattle, there are spots where it exists in the soil, and to which they resort to obtain it; such are the "buffalo-licks" of North America. phorus exists also in the yolk and white of the egg, and in milk, the substances on which the young animal subsists during the period of its most rapid growth; it abounds not only in many animal substances used as food, but also (in the state of phosphate of lime or bone-earth) in the seeds of many plants, especially the grasses; and in smaller quantities it is found in the ashes of almost every plant. When flesh, Phos bread, fruit, and husks of grain, are used as the chief articles 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 excretions. 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 phosphoric 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 sulphate, 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 carbonate 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 quantity 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 162 DIGESTION AND ABSORPTION. the blood, and is thrown out from the surface, mingled with animal matter. This hardens in a day or two, and the new covering is complete. The concretions in the stomach are then found to have disappeared; but they are gradually replaced, before the supply of lime they contain is again required. CHAPTER IV DIGESTION AND ABSORPTION. 171. HAVING now considered the nature of the food of Animals, and the sources from which it is obtained, we have next to consider the process by which the aliment is received into their bodies, and prepared to form a part of their own fabric. This process, termed Digestion, is naturally divided, among the higher animals at least, into various stages. In the first place, there is the prehension or laying hold of the aliment, and its introduction into the mouth or entrance to the digestive cavity. In the mouth it usually undergoes a preparation; which consists partly in its being cut, ground, or crushed, by mechanical action, into minute pieces; and partly in the working-up of these pieces with a fluid that is poured into the mouth,—the saliva. These two processes are termed mastication and insalivation; similar processes are performed, in some animals, in a part of the digestive tube intermediate between the mouth and the stomach, and even in the latter itself. The stomach is usually situated at some distance from the mouth, and is connected with a tube called the esophagus or gullet; and the passage of the food into this, constituting the act of swallowing, is termed deglutition. The food, having arrived in the stomach, is acted-upon by a peculiar fluid which it contains, and much of its alimentary portion is dissolved, so that a pulpy mass is formed which is termed chyme; hence this process, which is the first stage of digestion properly so called, is termed chymification or the manufacture of chyme. The chyme, which passes into the intestines, is further acted-on by secretions that are poured into them; and a certain nutritive combination of albuminous PREHENSION OF FOOD. 163 and fatty matters, termed chyle, is separated from the matters. that are to be thrown off: this process, which is the second stage of true digestion, is termed chylification. The rejected portions of the food, with secretions poured into the alimentary canal, find their way out through the intestinal tube; and are voided at its terminal orifice by the act of defecation. And lastly, the nutritive materials are taken up by absorption into vessels that are distributed upon the walls of the digestive cavity, and undergo a gradual change, by which they are converted into blood. These two processes are called absorption and sanguification (or manufacture of blood). Each of the foregoing stages will now be separately considered. Prehension of Food. 172. The introduction of aliment within the entrance to the digestive cavity is accomplished in various methods in different animals. In the Mammalia in general, the aperture of whose mouth is guarded by fleshy lips, these, with the jaws and teeth, are the chief instruments of this operation. But in Man and the Monkey tribe the division of labour is carried further; the food being laid hold of by the anterior members, or hands, and by them carried to the mouth. Where the hand has the power of grasping, and especially where the thumb can be opposed to the fingers, the action of a single member is sufficient; but there are several animals which, like the Squirrel, use both limbs conjointly to hold their food, the extremity not having itself the power of grasping. The Ant-eaters, Woodpeckers, Chameleons, and other insect-eating animals, obtain their food. by means of a long extensible tongue; this either serving to transfix the insect, or being covered with a viscid saliva which glues it to the surface. The Giraffe uses its long tongue to lay hold of the young shoots on which it browses; and the Elephant employs its trunk, which is nothing else than a prolonged nose, for every kind of prehension (fig. 82). Many of the Invertebrata are furnished with little appendages round their mouths by which the food is conveyed into them; such are the palpi Fig. 82 HEAD OF ELEPHANT. 164 RECEPTION OF SOLID AND LIQUID ALIMENT. of Insects, of which a pair is attached to each jaw (fig. 84); the tentacula of Mollusks, which are sometimes extremely prolonged, as in the Cuttle-fish tribe (fig. 85); and the similar organs of the polypes (fig. 71). 173. The reception of liquids is accomplished in two ways. Sometimes the liquid is made to fall into the mouth, simply by its own weight (fig. 86); in other instances it is drawn or pumped up into this cavity,-either by the expansion of the chest, which causes a rush of air towards the lungs, or by the movement of the tongue, which, being drawn back like a piston, produces the action of sucking. Some of the lower. animals are destined to be entirely supported by liquids which they find in plants, or which they draw from the bodies of other animals whereon they live as parasites. This is the case with many Insects; and their mouth, instead of presenting the ordinary structure, is formed into a sort of tube or trunk, very much extended, through which the juices are drawn up according to the wants of the animal. Such a conformation exists in the butterfly and moth tribe, whose |