ANIMAL ORGANIZATION. 89 



constituents. But substances of still greater hardness, united 

 with perfect rigidity, are wanted, in numberless instances 

 for giving effectual protection to soft and delicate structures, 

 for supplying a firm basis to the framework of the body, and 

 for constructing levers of various kinds to be employed in 

 the more energetic movements of the higher animals. For 

 all these purposes it was necessary to superadd a material 

 endowed with stronger cohesive powers, and capable by its 

 dense concretion of forming solid and Inflexible organs. 

 The substances which nature has selected for this office are 

 the salts of lime. Sometimes the Carbonate, and sometimes 

 the Phosphate of lime is employed for forming these hard 

 and unyielding structures; and often both these calcareous 

 substances are united together in different proportions in the 

 same solid fabric. When the carbonate of lime predomi- 

 nates, or is the sole earthy ingredient, it constitutes Shell: 

 when there is a greater proportion of the phosphate, it is 

 called a Crust, as is the case with the coverings of the lob- 

 ster and the crab: when the earthy matter consists almost 

 wholly of phosphate of lime, it composes the different forms 

 oiBone. I shall have occasion to describe the formation 

 and properties of each of these structures in the sequel. 



The protection of the delicate structure of the fabric from 

 the injurious influence of external agents is an object of 

 great imjDortance in the animal economy, and is one which 

 nature has shown extreme solicitude to secure. For this 

 purpose she has provided the integuments, under which 

 designation are included not merely the skin, but also all the 

 parts that are immediately connected with it, and are formed 

 and nourished by the same vessels. No parts of the animal 

 structure present greater diversity in their form and out- 

 ward appearance than the integuments; yet it is easy to dis- 

 cover, amidst all these varieties, that the same general plan 

 has been followed in their construction, and that each par- 

 ticular formation is the result of a combination of the same 

 elementary structures. Of these elements the most important, 

 and that which generally composes the chief bulk of the 

 skin, is the Corium, or true skin. The outermost layer is 

 Vol. I. 12 



