1020 VERTEBRATED ANIMALS 



form of the same thing, the walls of its ' tubuli ' and the ' inter- 

 tubular substance ' 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 Foramina/era we have seen a minutely tubular 

 structure to be formed around the individual threads of sarcode 

 which proceeded from the body of the contained animal. It may 

 now be asserted, indeed, that the bodies of even the highest animals 

 are everywhere penetrated by that protoplasmic 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 specialised or differentiated 

 tissues. As it is the purpose of this work, not to instruct the 

 professional student in histology (or the science of the tissues), 

 but to supply scientific information of general interest 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, 1 which gives support and protection to the softer parts of 

 the fabric. 



Bone. The microscopic characters of osseous tissue may some- 

 times 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 dis- 

 played more perfectly by artificial sections, the details of the arrange- 

 ment 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 extremity we find the outside wall 

 gradually becoming thinner, whilst the interior, instead of forming 

 one large cavity, is divided into a vast number 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 extends 

 throughout the shaft, which is not so completely differentiated into 

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

 In the most developed kinds of ' flat ' bones, again, such as those of 

 the head, we find the two surfaces to be composed of dense plates of 

 bone, with a ' cancellated ' structure between them ; whilst in the less 

 perfect type presented to us in the lower Yertebrata, the whole 

 thickness is usually more or less ' cancellated,' that is, divided up 

 into minute medullary cavities. When we examine, under a low 

 magnifying power, a longitudinal section of a long bone, or a section 



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

 internal skeleton, but also the hard parts protecting the exterior of the body, which 

 form the dermal skeleton. 



