CALCIFICATION OF FIBRES AND CELLS. 



161 



Fig. 59. 



Calcified Areolar Structure, of which the Skele- 

 1on of the Echinodermata is composed ; from the 

 Spine of an Echinus. Magnified 150 diameters. 



Fig. 60. 



9. Tissues consolidated by Earthy deposit. Bones and Teeth. 



192. Both the Fibres and Cells of the Animal tissue, there is reason to be- 

 lieve, may be consolidated by mineral deposits ; these being chemically united 

 with the Gelatine of the Fibres ; or secreted, either alone, or in combination 

 with gelatine, into the cavities of the 

 Cells, by their own inherent powers. 

 We have an example of the form- 

 ation of a skeleton by the consoli- 

 dation of fibres, in the shell and 

 other hard parts of the Echinoder- 

 mata ; the intimate structure of which, 

 as shown by the Microscope, strong- 

 ly reminds us of Areolar tissue that 

 might have undergone the calcifying 

 process. Again, we have an exam- 

 ple of the formation of a skeleton 

 by the deposit of mineral matter in 

 the cavities of cells, in the shells of 

 Mollusca; in many of which (espe- 

 cially among the Bivalves) the cellu- 

 lar character is permanently shown, 

 a consistent membrane being left, 



after the Carbonate of Lime that consolidated the cell has been dissolved 

 away by an acid. An arrange- 

 ment precisely similar, as regards 

 the animal constituent, is found 

 in the Enamel of Teeth ( 215) ; 

 the only difference being in the 

 consolidating material, which is 

 chiefly the Phosphate of Lime, 

 a mineral far harder than the 

 Carbonate. It is not always, 

 however, that the original cells 

 preserve their character so dis- 

 tinctly; for it is very commonly 

 found, that they have coalesced 

 with each other, in such a man- 

 ner as not to be distinguishable 

 in the fully-formed tissue. We 

 also frequently observe, in the 

 skeletons of Vertebrata, that the 



whole substance is not consolidated, but that cavities and channels are left in 

 it ; which seem destined to perform some office connected with the interstitial 

 changes, that continue to take place in the tissues subsequently to their first 

 formation. It has been already pointed out ( 5), that the internal bony ske- 

 letons of Vertebrated animals are destined to undergo a degree of interstitial 

 change (in order to adapt them to the progressive growths of the parts that 

 cover them), which is not required in the external envelopes of Invertebrated 

 animals ; these being capable of sufficient enlargement by addition to their 

 edges merely ; or else being periodically thrown off, and renewed upon a 

 larger scale. It is obvious that, if the whole substance be consolidated by 

 calcareous deposit, there can be no permeation of nutritive fluid through it ; 

 but, on the other hand, if it be traversed by tubuli, commencing from the near- 

 est vascular surface ; or if a series of minute chambers, connected by still 



14* 



Cellular membrane, left after the removal of the Cal- 

 careous matter from the shell of Pinna. Magnified 165 

 diameters. 



