HISTOLOGY OF EPITHELIAL AND CONNECTIVE TISSUES. 35 



Retiform and adenoid tissue are also modifications of areolar tissue. 

 The meshes of the former contain but little ground substance, its place being 

 taken by fluid; the meshes of the latter contain large numbers of lymph 

 corpuscles. 



Fibrous Tissue. This variety of connective tissue is widely distributed 

 throughout the body. It constitutes almost entirely the ligaments around 

 the joints, the tendons of the muscles, the membranes covering organs such 

 as the heart, liver, nervous system, bones, etc. All fibrous tissue, wherever 

 found, can be resolved into elementary bundles, which on microscopic ex- 

 amination are seen to consist of delicate, wavy, transparent, homogeneous 

 fibers, which pursue an independent course, neither branching nor uniting 

 with adjoining fibers. A small amount of ground substance serves to hold 

 them together. Fibrous tissue is tough and inextensible, and in consequence 

 is admirably adapted to fulfil various mechanical functions in the body. 

 It is, however, quite pliant, bending easily in all directions. When boiled, 

 fibrous tissue yields gelatin, a derivative of collagen. 



Elastic Tissue. The fibers of elastic tissue are usually associated in 

 varying proportions with the white fibrous tissue; but in some structures 

 as the ligamentum nuchae, the ligamenta subflava, the middle coat of the 

 larger blood-vessels the elastic fibers are almost the only elements present, 

 and give to these structures a distinctly yellow appearance. The fibers 

 throughout their course give off many branches, which unite with adjoining 

 branches to form a more or less close network. As the name implies, these 

 fibers are highly elastic, and are capable of being extended as much as sixty 

 per cent, before breaking. 



Cartilaginous Tissue. This form of connective tissue differs from 

 the preceding varieties chiefly in its density. As a rule, it is firm in con- 

 sistency, though somewhat elastic. It is opaque, bluish-white in color, 

 though in thin sections translucent. All cartilaginous tissues consist of 

 connective-tissue cells embedded in a solid ground substance. According 

 to the amount and texture of the ground substance, three principal varieties 

 may be distinguished: 



i. Hyaline cartilage, in which the cells, relatively few in number, are em- 

 bedded in an abundant quantity of ground substance. The body of 

 the cells is in many instances distinctly marked off from the surround- 

 ing substance by concentric lines or fibers, which form a capsule for the 

 cell. Repeated dvision of the cell substance takes place, until the whole 

 capsule is completely occupied by daughter cells. The ground substance 

 is pervaded by minute channels, which communicate on one hand with 

 the spaces around the cells, and on the other with lymph-spaces in the 



