NATURE OF THE DIGESTIVE APPARATUS. 325 



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. Phosphorus 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 ; and 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. In 

 smaller quantities it is found in the ashes of almost every plant. When flesh, 

 bread, fruit, and husks of grain, are used as the chief articles 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 com- 

 pounds 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 vege- 

 table substances used as food by Man, such as potatoes, cabbage, peas, cucum- 

 bers, mustard, &c.; and probably in most articles from which other animals 

 derive their support. Lime is one of the most universally diffused of all mineral 

 bodies ; for there are 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 ; and in this state it exists largely 

 in the seeds of most grasses, especially in wheat-flour. If it were not for their 

 deficiency in Phosphate of Lime, beans and peas would be more nutritious 

 than wheaten-flour, the proportion of azotized matter they contain being much 

 larger. A considerable Quantity of lime exists, in the state of carbonate and 

 sulphate, in all hard water. 



435. The introduction of alimentary matter into the system, is accomplished 

 in Animals by the reception of food into an internal cavity, where it is sub- 

 jected to a preparatory process, to which nothing analogous exists in Plants, 

 and which is termed Digestion. This process may be said to have" three 

 different purposes in view ; the reduction of the alimentary matter to a fluid 

 form, so that it may become capable of absorption ; the separation of that 

 portion of it which is fit to be assimilated or converted into organized texture, 

 from that which cannot serve this purpose, and which is at once rejected ; 

 and the alteration (when required) of the chemical constitution of the former, 

 which prepares it for the important changes it is subsequently to undergo. 

 The simplest conditions requisite for the accomplishment of these purposes 

 are the following : a fluid capable of performing the solution and of effecting 

 the required chemical changes ; a fluid capable of separating the unorgan- 

 izable matter, by a process analogous to chemical precipitation ; and a cavity 

 or sac, in which these operations may be performed. In the lowest Animals, 

 we find this cavity formed on a very simple plan ; being evidently nothing 

 else than an inversion of the external integument, communicating with the 

 exterior by one orifice only, through which the food is drawn in, and the 

 excrementitious matter rejected. The fluid necessary to dissolve the food, 

 which is known by the name of gastric fluid or juice, and that required to 

 separate the portion which is to be thrown off, which is known as the bile, are 

 secreted in the walls of the stomach. In the Sea-Anemone, which affords a 

 very characteristic example of this type of structure, it cannot be ascertained 

 that the very rapid solution of food, which takes place in the digestive cavity, 

 is assisted by any movement of its walls. In Polypes of a higher con- 

 formation, however, the digestive cavity is provided with a second orifice; 

 the stomach opens into an intestinal tube, through which the excrement is 

 rejected in little pellets; and the food, before entering the true digestive 

 cavity, is submitted to a powerful gizzard or triturating apparatus. Still the 

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