CHAPTEE XVIII. 



VBRTBBRATBD ANIMALS. 



402. We are now arrived at that highest division of the Ani- 

 mal Kingdom, in v^hich the bodily fabric attains its greatest 

 development, not only as to completeness, but also as to size ; 

 and it is in most striking contrast with the class we have been 

 last considering. Since not only the entire bodies of Verte- 

 brated animals, but, generally speaking, the smallest of their 

 integral parts, are far too large to be viewed as Microscopic 

 objects, we can study their structure only by a separate exami- 

 nation of their component elements ; and it seems, therefore, to 

 be a most appropriate course, to give, under this head, a sketch 

 of the microscopic characters of those primary tissues, of which 

 their fabric is made up, and which, although they may be traced 

 with' more or less distinctness in the lower tribes of Animals, 

 attain their most complete development in this group. Since 

 the time when Schwann first made public the remarkable results 

 of his researches (p. 56), it has been very generally believed that 

 all the Animal tissues are formed, like those of Plants, by a 

 metamorphosis of Cells; an exception being taken, however, by 

 some Physiologists, in regard to the simple fibrous tissues (§ 417). 

 The tendency of many recent investigations, however, has been 

 to throw further doubt on the generality of this doctrine ; since 

 they appear to indicate that many other tissues than the fibrous 

 may be formed (like these) by the consolidation of the plasma or 

 formative fluid, without passing through the intermediate condi- 

 tion of cells. Hence 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 micro- 

 scopic examination ; and as it is of no essential consequence what 

 order is adopted, we may conveniently begin with the structure 

 of the skeleton,^ which gives support and protection to the softer 

 parts of the fabric. , 



403. Bone. — The Microscopic characters of osseous tissue may 



' 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 

 forms the dermal skeleton. 



