102 THE HUMAN BODY. 



ably parallel to one another in a wavy course (Fig. 46) and 

 never branch or unite. Their diameter varies from 0.0005 to 

 0.001 millimeter (-5-00-00- to ^-jhrTr f an inch). 



Chemically this tissue is characterized by the fact that its 

 fibres swell up and become indistinguishable when treated 

 with dilute acetic acid, and by the fact that it yields gelatin 

 when boiled in water. The substance in it, called ossein in 

 bones, which is turned into gelatin by such treatment, is here 

 known us i.oU'.tgen. Glue is impure gelatin obtained from 

 tendons and ligaments, and calf s-foot jelly, so often recom- 

 mended, to invalids, Is a purer form of the same substance 

 oiriained by boiling' the feet of calves, which contain the ten- 

 dons of many muscles passing from the leg to the foot. 



Elastic Tissue. This is almost invariably mixed in some 

 proportion in all specimens of white fibrous tissue, even the 

 purest, such as the tendons of muscles; but in certain places 

 it exists almost alone, as for example in the ligaments (liya- 

 menta sub/lava) between the arches of the vertebrae, and in 

 the coats of the larger arteries. In quadrupeds it forms the 

 great ligament already referred to (p. 83), which helps to sus- 

 tain the head. This tissue, in mass, is of a dull yellow color 

 and extremely extensible and elastic; when purest nearly as 

 much so as a piece of india-rubber. Sometimes it appears 

 under the microscope to be made up of delicate membranes, 

 but more often it is in the form of fibres (Fig. 46) which are 

 coarser than those of white fibrous tissue and frequently 

 branch and unite. It is unaffected by acetic acid and does 

 not yield gelatin when boiled in water. 



Connective-tissue Corpuscles. The fibres of white fi- 

 brous tissue, wherever it is found, are united into bundles by 

 a structureless ground-material known as the cement-sub- 

 stance, which also invests each bundle, or skein as we may 

 call it, with a delicate coating. In this ground-substance are 

 numerous cavities, branched and flattened in one diameter, 

 and often intercommunicating by their branches. In these 

 cavities lie nucleated masses of protoplasm (Fig. 47), fre- 

 quently also branched, known as the connective-tissue cor- 

 puscles. These it is which build up the tissue, each 

 cell in the course of development forming around it a 

 quantity of intercellular substance, which subsequently be- 

 comes fibrillated in great part, the remainder forming the 

 cement. The cells do not quite fill the cavities in which they 



