66 



HISTOLOGY 



by the acid These rings have been observed by Ranvier in living con- 

 nective tissue fibers, and it is therefore improbable that they are remnants 

 of a sheath which surrounded the entire fiber, as some have thought. 

 They are probably formed of elastic substance. 



In addition to the white or collagenous fibers, connective tissue con- 

 tains fibers of a second sort, known as elastic fibers. They are absent from 

 corneal tissue, the mucous tissue of the umbilical cord and generally, 

 though not always, from reticular tissue. Since they develop later than 

 the white fibers, they are not found in young connective tissue; but other- 

 wise they are present, though varying 

 greatly in abundance, in all forms of con- 

 nective tissue. They are not destroyed 

 by dilute acids or alkalies, and are de- 

 scribed as composed of elastin, an albumi- 

 noid body which does not yield gelatin on 

 boiling. Unlike the white fibers they are 

 not composed of smaller elements or fibrils, 

 but each fiber is a structureless homogen- 

 eous thread. In favorable cases, however, 

 an enveloping sheath may be seen. In 

 tissue which has not been torn apart the 

 elastic fibers form a net (Fig. 53, A). The 

 fibers meet and fuse with one another; and 

 across the angles thus formed, one or two 

 delicate strands are commonly to be found. When the tissue is pulled 

 apart so that the net is broken, the fibers kink and recoil like tense wires 

 (Fig. 52, e). 



The origin of the elastic fibers has not been determined. They have 

 been said to arise within the cells by the fusion of granules of elastin. 

 Mall's idea of their exoplasmic origin is illustrated by their relation to the 

 cells in Fig. 53, B. Others consider that they are formed from the inter- 

 cellular substance. 



Although elastic fibers are clearly seen in fresh connective tissue, 

 they are often invisible in specimens stained with haematoxylin and eosin. 

 In order to determine their presence, sections may be stained with re- 

 sorcin-fuchsin, which leaves the white fibers nearly colorless, but makes 

 the elastic fibers dark purple; or other special stains may be used. In 

 some situations, however, the elastic tissue is highly developed and may 

 be seen with any stain. This is true of the fenestrated membranes found 

 in many blood vessels. A fenestrated membrane is a network of elastic 

 fibers in which the fibers are so broad that they appear to form a perfor- 

 ated plate (Fig. 54, A). The greatest development of elastic tissue prob- 

 ably occurs in the ligament of the neck in grazing animals, which consists 



A B 



FIG. 53. 



A, Elastic fibers of the subcutaneous areo- 

 lar tissue of a rabbit. (After Schafer.) 

 B, Cells in relation with elastic fibers, 

 after treatment with acetic acid. Sub- 

 cutaneous tissue of a pig embryo. 

 (After Mall.) 



