THE CONNECTIVE TISSUES. 



The important group of connective substances, the most widely distributed of 

 all tissues, is the direct product of the great mesoblastic tract ; the several members 

 of this extended family are formed by the differentiation and -specialization of the 

 intercellular substance wrought through the more or less direct agency of the meso- 

 blastic cells. The variation in the physical characteristics of the connective tissues 

 is due to the condition of their intercellular constituents. During the period of em- 

 bryonal growth these latter are represented by gelatinous, plastic substances ; a 

 little later by the still soft, although more definitely formed, growing connective 

 tissue, which, in turn, soon gives place to the yielding, although strong, adult 

 areolar tissue. 



Grouped as masses in which white fibrous tissue predominates, the intercellular 

 substance presents the marked toughness and inextensibility of tendon ; where, on 

 the contrary, large quantities of yellow elastic tissue are present, extensibility is 

 conspicuous. Further conden- 

 sation of the intercellular sub- F'G. 93. 

 stance produces the resistance 

 encountered in hyaline carti- 

 lage, intermediate degrees of 

 condensation being presented 

 by the fibrous and elastic varie- 

 ties. In those cases in which 

 the ground-substance becomes 

 additionally impregnated with 

 calcareous salts, the well-known 

 hardness of bone or dentine is 

 attained. Notwithstanding these 

 variations in the density of the 

 intercellular substance, the cel- 

 lular elements have undergone 

 but little change, the connective- 

 tissue corpuscle, the tendon-cell, 

 the cartilage-cell, and the bone- 

 corpuscle being morphologically 

 identical. 



The principal forms in which the connective substances occur may be grouped as 

 follows : 



1. Imynahire connective tissue, as the jelly of Wharton in the umbilical cord 

 and the tissues of embryos and of young animals. 



2. Areolar tissue, forming the subcutaneous layer and filling intermuscular 

 spaces, and holding in place the various organs. 



3. Dense fih-o-elastic tissue, found in the fasciae, the sclera, the ligaments, etc. 

 Where white fibrous tissue predominates and yellow elastic tissue is pra,ctically 

 wanting, structures of the character of tendon or of the cornea are produced ; where, 

 on the other hand, elastic tissue is in excess of fibrous tissue, highly extensible 

 structures, as the ligamentum nuchae or the ligamenta subflava, result. 



4. Cartilage, fibrous, elastic, and h^'aline varieties. 



5. Bone and dentine, in which impregnation of lime salts contributes character- 

 istic hardness. 



6. Reticulated coyinective tissue, occurring as the supporting framework in the 

 lymphatic tissues, and as the interstitial reticulum of many organs. 



7. Adipose tissue. 



The Cells of Connective Tissue. — The cellular elements of the connective 



73 



Embryonal connective-tissue cells from the umbilical cord. X 500. 



