CHAPTER III. 

 TIIK rDXXF/TIVF. TISSFES. 



In contrast to epithelial tissues, where intercellular material is ab- 

 sent or insignificant in amount, in connective tissues the emphasis is 

 placed upon the nature of the intercellular material. The cells of con- 

 nective tissues usually form a small part of the tissue mass and are 

 partially or completely separated from each other by the various 

 types of intercellular material which they form. This emphasis 

 upon the non-cellular, intercellular products of connective tissues 

 is directly associated with the role they play. In general, connective 

 tissues form the supporting structures of the body, from the heavy 

 framework of the bony skeleton to the finer networks supporting the 

 capillaries. In addition to the formation of intercellular products 

 some of the cellular components of the tissues are active in storage 

 and phagocytosis. The various t^-pes of connective tissues trace 

 their origin back to the mesoderm of early stages of embryonic 

 development. 



Classification.— As one might expect from the nature of connective 

 tissues, classification of the various t>-pes is based both upon the 

 cellular constitution and the nature and arrangement of the inter- 

 cellular deposits. There are intergrading forms and also some 

 variation in the general characteristics considered as distinguishing 

 the t\"pes. so that here, as in the case of epithelia, there may be 

 difficulty in classification. Two t^-pes, mesenchyme and mucous 

 tissue, are mainly embryonic. In the adult vertebrate, the following 

 t^npes are distinguished: reticular tissue, loosely organized fibro- 

 elastic tissue, adipose tissue, densely organized fibrous and elastic 

 tissue, cartilage, and bone. These X\"pes include modifications or 

 subdivisions which may be best considered in order when describing 

 the features of each tx-pe. 



MESENCHYME. 



As soon as the three germ layers are well established in the 

 embryonic development, cells begin moving away from the meso- 

 derm to occupy spaces between it and the ectoderm and endo- 

 derm. These early, wandering cells compose mesenchyme tissue. 



