VERTEBRATED ANIMALS. 255 



rounded by the product of its own elaborating action, those which form 

 the 'connective-tissue-corpuscles' are connected together by radiating 

 prolongations (Fig. 461) that pass between the fibres, so as to form a 

 continuous network closely resembling that formed by the pseudopodia 

 of the Rhizopod (Fig. 283). Of this we have a most beautiful example 

 in Bone; for whilst its solid substance may be considered as connective 

 tissue solidified by calcareous deposit, the ' lacunae' and 'canaliculi' 

 which are excavated in it (Fig. 441) give lodgment to a set of radiating 

 corpuscles closely resembling those just described; and these are centres 

 of 'germinal matter/ which appear to have an active share in the for- 

 mation and subsequent nutrition of the osseous texture. In Dentine (or 

 tooth-substance) we seem to have another form of the same thing; the 

 walls of its 'tubuli' and the ' intertubular substance' ( 655) being the 

 'formed material' that is produced from thread-like prolongations of 

 ' germinal matter ' issuing from its pulp, and continuing during the life 

 of the tooth to occupy its tubes; just as in the Foraminifera we have 

 seen a minutely-tubular structure to be formed around the individual 

 threads of sarcode which preceded from the body of the contained ani- 

 mal (Figs. 314, 335). It may now be stated, indeed, with considerabk 

 confidence, that the bodies of even the highest Animals are everywhere 

 penetrated by that sarcodic substance of which those of the lowest and 

 simplest are entirely composed; and that this substance, which forms 

 a continuous network through almost every portion of the fabric, is the 

 main instrument of the Formation, Nutrition, and Reparation of the 

 more specialized or differentiated Tissues. As it is the purpose of this 

 work not to instruct the professional student in Histology (or the Sci- 

 ence of the Tissues), but to supply scientific information of general inter- 

 est to the ordinary Microscopist, no attempt will here be made to do more 

 than describe the most important of those distinctive characters which 

 the principal tissues present when subjected to Microscopic examination; 

 and as it is of no essential consequence what order is adopted, we may 

 conveniently begin with the structure of the skeleton, l which gives sup- 

 port and protection to the softer parts of the fabric. 



652. Bone. The Microscopic characters of osseous tissue may some- 

 times may be seen in a very thin natural plate of bone, such as in that 

 forming the scapula (shoulder-blade) of a Mouse; but they are displayed 

 more perfectly by artificial sections, the details of the arrangement being 

 dependent upon the nature of the specimen selected, and the direction 

 in which the section is made. Thus when the shaft of a 'long' bone of 

 a Bird or Mammal is cut-across in the middle of its length, we find it to 

 consist of a hollow cylinder of dense bone, surrounding a cavity which 

 is occupied by an oily marrow; but if the section be made nearer its ex- 

 tremity, we find the outside wall gradually becoming thinner, whilst the 

 interior, instead of forming one large cavity, is divided into a vast num- 

 ber of small chambers, partially divided by a sort of ' lattice-work ' of 

 osseous fibres, but communicating with each other and with the cavity 

 of the shaft, and filled, like it, with marrow. In the bones of Reptiles 

 and Fishes, on the other hand, this ' cancellated ' structure usually ex- 

 tends throughout the shaft, which is not so completely differentiated 

 into solid bone and medullary cavity as it is in the higher Vertebrata. 



1 This term is used in its most general sense, as including not only the proper 

 vertebral or internal skeleton, but also the hard parts protecting the exterior of 

 the body, which form the dermal skeleton. 



