Cellular Organization of Plants and Animals 49 



B. Connective (Supportive) Tissues (Figs. 12 and 13) 



1. RETICULAR 



Fine white fibers in the form of a network for supporting 

 the cells of other tissues in certain organs; examples — 

 spleen, liver, lymph glands, etc. 



2. FIBROUS 



(a) Areolar — Minute white fibers frequently in bundles 

 which form a network with a homogeneous, ground 

 substance in which are scattered the connective tissue 

 cells (rounded, irregular, or spindle shaped) ; may 

 contain thicker, single, yellow, elastic fibers; found 

 surrounding muscles, nerves, etc. 



(b) Tendons and ligaments — White fibers which run 

 parallel to each other, with cells between them; ten- 

 dons connect muscles to bones; ligaments connect 

 bones to bones. 



(c) Elastic (yellow) — A preponderance of single, thick, 

 yellow, elastic fibers over the less numerous white 

 fibers; found in the blood vessel walls, vocal folds, 

 lungs, etc. 



3. ADIPOSE 



Certain rounded cells are filled with fat globules of various 

 sizes; found in various places beneath the skin, around 

 certain organs, etc. 



4. CARTILAGE 



(a) Hyaline ("gristle") — This consists of a clear, firm, 

 gelatinous, homogeneous matrix with scattered spaces 

 called lacunae (la-ku'na) (L. lacuna, cavity) in 

 which are one or more rounded cartilage cells which 

 secrete the cartilage; examples — ends of long bones 

 and ribs, nose, trachea (windpipe), etc. 



(b) Fibrocartilage — Numerous fibers are present in the 

 matrix of this cartilage; examples — external ear, be- 

 tween the vertebrae, etc. 



5. BONE 



The ground substance, or matrix, is hardened with calcium 

 carbonate and calcium phosphate; bone cells are pres- 

 ent in the lacunae which are connected with fine 

 canals. The units of bone construction, known as 

 Haversian systems, consist of (a) a central canal with 

 an artery, vein, and nerve; (b) lamellae made of layers 

 of bony flakes so arranged as to form rough-walled 

 canals which are arranged concentrically in circles or 

 ovals, around the central canal; (c) the lacunae or 

 enlarged spaces associated with the lamellae and con- 

 taining the irregularly shaped bone cells; (d) the tiny, 

 wavy, canal-like canaliculi which radiate from the 

 lacunae and connect the lacunae with each other and 

 with the central canal; (e) the hard matrix (bone) 

 which occupies spaces not previously described and is 

 secreted by the bone cells by the incorporation of lime 

 salts. 



Bones protect, support, assist in locomotion, serve for at- 

 tachment of muscles and other tissues, assist in hearing 

 (ear bones), etc. 



