152 



COMPARATIVE ANATOMY 



resistance to tensile strain. The essential mechanical structures in such 

 tissue are relatively coarse white fibers (Fig. 114) consisting of an albumi- 

 noid substance, collagen, the source of gelatin and glue. These collagenous 



Fig. 114. — Subcutaneous connecuve libsue from a cat. Highly magnified. The 

 fibers were artificially separated and examined, unstained, in water, except the fiber 

 marked "a" which was treated with dilute acetic acid, a, c, white fibers; b, fat cell; 

 d, connective-tissue cell; e, elastic fibers. (From Bremer, Text-book of Histology.) 



fibers are only slightly elastic. They may be branched. Each fiber is a 

 bundle of very delicate fibrils. Exceedingly flattened cells with flat 



nuclei appear as if clinging closely to the 

 surface of a fiber. These connective-tissue 

 cells or fibrocytes are presumably the 

 agencies which have brought about the 

 production of the fiber. 



Elastic fibers (Fig. 114, e) are much 

 finer than collagenous fibers and differ from 

 them chemically in being composed of 

 elastin which is not a source of gelatin. An 

 occasional elongated fibrocyte may be seen 

 stretching along the surface of a fiber 

 (Fig. 115). Elastic fibers commonly occur 

 intermingled with collagenous fibers. The 

 elasticity of a connective tissue as a whole 

 depends upon the relative abundance of 

 elastic fibers in it. 



Connective tissue forming a loose open 

 mesh-work, as does the subcutaneous tissue 

 lying between the skin and the muscle of 

 the body, is called areolar tissue. 



Reticular tissue is composed of cells whose branched processes join 

 to form a network. Associated with the processes are compara- 

 tively short fibers which seem to be similar to collagenous fibers. 



Fig. 115. — A, elastic fibers of 

 the subcutaneous areolar tissue of 

 a rabbit. B, cells related to 

 elastic fibers, as seen after treat- 

 ment with acetic acid; from sub- 

 cutaneous tissue of a pig embryo. 

 (From Bremer, Text-book of 

 Histology: A, after Schafer; B, 

 after Mall.) 



