252 OF FOOD AND THE DIGESTIVE PROCESS. 



allowed to run to waste, so long will it be necessary that the requisite 

 amount of phosphate of lime should be drawn from foreign sources. 



441. The phosphate of lime, as already mentioned, seems to perform 

 important offices of a chemical nature in the animal economy, besides 

 being the chief solidifying ingredient of bones and teeth ; but the car- 

 bonate would seem principally destined to mechanical uses only ; and 

 we find it predominating, or existing as the sole mineral ingredient, in 

 those non-vascular tissues of the Invertebrated animals, which give sup- 

 port and protection to their soft parts ( 277). The degree of develop- 

 ment of these tissues depends in great part upon the supply of carbonate 

 of lime which the animals receive. Thus the Mollusca which inhabit 

 the sea, find in its waters the proportion of that substance which they 

 require ; but those dwelling in streams and fresh-water lakes, which 

 contain but a small quantity of lime, form very thin shells ; whilst the 

 very same species inhabiting lakes, which, from peculiar local causes, 

 contain a large impregnation of calcareous matter, form shells of re- 

 markable thickness. The Crustacea which periodically throw off their 

 calcareous envelope ( 285), are enabled to renew it with rapidity by a 

 very curious provision. There is laid up in the walls of their stomachs 

 a considerable supply of calcareous matter, in the form of little concre- 

 tions, which are commonly known as " crabs' eyes." When the shell is 

 thrown off, this matter is taken up by the circulating current, and is 

 thrown out from the surface, mingled with the animal matter of which 

 the shell is composed. 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 drawn upon. The large amount of carbo- 

 nate of lime which is required by the laying Hen, is derived from chalk, 

 mortar, or other substances containing it, which she is compelled by her 

 instinct to eat ; 'and if the supply of these be withheld, the eggs which 

 she deposits are soft on their exterior, not being destitute of shell, as 

 commonly supposed, but having the fibrous element of the shell ( 181) 

 unconsolidated by the intervening deposit of chalky particles. 



2. Of the Digestive Apparatus, and its Actions in general. 



442. It has been already pointed out, that the nature of the food of 

 Animals is so far different from that of Plants, as to require the prepa- 

 ratory process of Digestion, before its nutritious part can be taken up 

 by the absorbent vessels and received into the system. 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 assi- 

 milated or converted into organized texture, from that which cannot 

 serve this purpose, and which is at once, rejected ; and the alteration, 

 to a certain extent 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 excrementitious matter, by a process analogous to 



