42 THE HUMAN BODY 



certain structures, where a high degree of elasticity is required, 

 elastic fibers make up the entire connective tissue content. Ex- 

 amples of this sort are the walls of the large arteries and the liga- 

 ments connecting the vertebrae. In quadrupeds these fibers form 

 the great ligament which helps to sustain the head (see p. 65) . 

 Elastic tissue is yellow in color, and consists chemically of an al- 

 buminoid, elastin, which in some important respects differs from 

 the albuminoid of the white fibers. 



Connective tissue fibers are not living structures. They owe 

 their origin to certain living cells, the so-called connective tissue 

 cells, which lie irregularly interspersed wherever connective tissue 



Fio. 13. Connective tissue cells: a, from areolar tissue; b, from tendon. 



fibers occur (Fig. 13). In areolar tissue many of the cells have 

 given up their function of forming fibers and have devoted them- 

 selves instead to storing within their substance masses of fat. 

 Adipose tissue consists of cells of this sort (Fig. 49), and occurs 

 in regions where areolar tissue is most abundant, as just under 

 the skin or in masses about certain internal organs. 



Temporary and Permanent Cartilages. In early life a great 

 many parts of the supporting framework of the Body, which after- 

 wards become bone, consist of cartilage. Such for example is the 

 case with all the vertebra?, and with the bones of the limbs. In 

 these cartilages subsequently the process known as ossification 

 takes place, by which a great portion of the original cartilaginous 

 model is removed and replaced by true osseous tissue. Often, 

 however, some of the primitive cartilage is left throughout the 

 whole of life at the ends of the bones in joints where it forms the 

 articular cartilages; and in various other places still larger masses 

 remain, such as the costal cartilages, those in the external ears 

 forming their framework, others finishing the skeleton of the nose 

 which is only incompletely bony, and many in internal parts of 

 the Body, as the cartilage of "Adam's apple," which can be felt 

 in the front of the neck, and a number of rings around the wind- 

 pipe serving to keep it open. These persistent masses are known 



