192 
On the Structure and Development of 
often substituted for another form in another animal, so that fibrous 
tissue or cartilage in one animal will perform the same function as 
bone in another, and they have in this way come to be regarded as 
equivalents of one another in a morphological sense ; again, it is 
quite apparent that one form often succeeds another form in the 
natural life of the body, fibrous tissue or cartillage of youth being 
transformed into bone in the adult. In the growths of tumours, 
these changes are frequently seen. 
The word connective tissue, originally proposed by Johannes 
Miiller, as distinguished from connective substance, has also some- 
times been applied to one or more members of the same class, and, 
indeed, it is in this way that much confusion has been produced, 
for while some observers have used the word in the broad sense of 
connective substance, others have limited it to some specific form, 
such as fibrous tissue (fibrillated connective tissue). 
To avoid any such source of error, we shall call each form by 
its distinctive name, as mucous tissue, adenoid tissue, and the like, 
and then we shall find that, though there is a strong bond of 
relation between all the forms, they (many of them) show as 
distinctive difierences as any other tissues in the body. The word 
connective tissue will accordingly be avoided entirely except where 
its character is specifically described, as when using the expression 
" fibrillated connective tissue," or " connective tissue of the kidney," 
&c. We may then expect to get more precise notions of the 
minute structure of each variety, and so of the peculiar relations 
they each hold to pathological change. 
The three that stand at the head of the list, viz. hone, carti- 
lage, and dentine, are in many respects better understood than the 
others, chiefly because in gross appearance they show distinctive 
differences and because their anatomical elements have been more 
easy to isolate. The consideration of them, however, does not 
come within the range of the present work, and no further mention 
of them will therefore be made. 
Our knowledge of connective substances dates from a compara- 
tively recent period, for the first systematic efforts to determine 
their minute structure appear to have been made by Schwann in 
1839. Since that time the doctrines in these matters have under- 
gone important modifications, and it will be essential to consider 
the more important of them before we can get a clear conception 
of the views which are now entertained. 
Schwann was the first to point out in these tissues certain 
bodies that he called spindle-shaped or caudate cells. The word 
*'ceir' is here used by Schwann in speaking of the variously 
shaped fixed cells, as distinguished on the one hand from the 
wandering cells which are now called leucocytes or lymphoid 
corpuscles, and on the other from the intercellular substance. The 
