MUCOUS TISSUE. 49 



Wandering cells, cells which are free and capable of moving 

 about in areolar tissue, are really leukocytes or lymph- eel Is, 

 and belong to the class of free cells. 



The white fibres consist of very fine colorless fibrillae 

 cemented together in coarser fibres (Fig. 16). They may be 

 arranged in an interlacing network or parallel in bundles, and 

 often present a wavy appearance. The fibrils do not branch. 

 They swell and become indistinct when treated with acetic 

 acid, often showing constrictions. They yield gelatin on 

 boiling. They are characterized by unyielding strength, and 

 occur in tendons, ligaments, etc., where tenacity and firmness 

 are required. 



The yellow elastic fibres (Fig. 17) are yellowish, highly 

 refractive, branching fibres, fine or coarse, forming bundles or 

 membranous interlacing networks. They yield elastin, not 

 gelatin, and stain differentially with orcein. When divided, the 

 freed ends curl up. They are characterized by elasticity, and 

 are widely distributed, occurring in especial abundance in the 

 ligamentum nuchae of animals, ligamenta subflava, blood- 

 vessels, and in yellow elastic cartilage. 



Two theories are held as to the development of the fibres, 

 white and yellow : one is that they are produced by a 

 transformation of the protoplasm of the connective-tissue 

 cells ; the other, probably more generally true, is that they 

 are not transformed cells, but develop in the mucinous inter- 

 cellular matrix by a sort of secretory process, under the 

 agency of the cells. 



Varieties of connective tissues: According to the varving 

 forms and proportions of the constituent elements, there are 

 many varieties of connective tissues, merging imperceptibly 

 into one another without sharp dividing lines. The following 

 best-marked types may be mentioned : 



Mucous tissue; Tendons; * 



Areolar tissue ; Retiform tissue ; 



White fibrous tissue ; Cartilage ; 



Yellow elastic tissue ; Bone ; 



Membranes ; Dentine ; 



Adipose tissue ; Neuroglia. 



Mucous tissue (Fig. 18) consists of mucous cells, flattened 



4 Hist. 



