CELLS AND TISSUES 57 



firm but elastic matrix secreted by cartilage cells which become em- 

 bedded in the matrix (Fig. 3.12). These cartilage cells are alive; they 

 may secrete collagenous fibers or elastic fibers to strengthen the cartilage. 



Bone consists of a dense matrix composed of proteins and calcium 

 salts identical with the mineral aragonite, Ca3(P04)2'CaC03. About 65 

 per cent of the bone is made of this mineral. The bone cells (osteoblasts) 

 secrete both the protein and the calcium salts. The osteoblasts become 

 surrounded and trapped by their own secretion and remain in micro- 

 scopic cavities (lacunae) in the bone as living osteocytes (Fig. 3.12). The 

 protein is laid down as minute fibers which contribute strength and 

 resiliency and the mineral salts contribute hardness to bone. As one 

 grows older the proportion of organic material in the bone gradually 

 decreases; hence the bones of elderly people are more brittle than those 

 of youth. 



At the surface of each bone is a thin fibrous layer called the peri- 

 osteum (peri, around; osteum, bone) to which the muscles are attached 

 by tendons. The periosteum contains cells, some of which differentiate 

 into osteoblasts and secrete protein and salts to bring about growth 

 and repair. Most bones are not solid, but have a marrow cavity in the 

 center. The apparently solid matrix of the bone is pierced by many 

 microscopic channels (Haversian canals) in which lie blood vessels and 

 nerves to supply the bone cells. The bony matrix is deposited, usually 

 in concentric rings or lamellae, around these Haversian canals. Each 

 bone cell is connected to the adjacent bone cells and to the Haversian 

 canals by protoplasmic processes of the bone cells which lie in minute 

 canals (canaliculi) in the matrix. The bone cells obtain oxygen and 

 raw materials and eliminate wastes by way of these canaliculi. The de- 

 tails of the architecture of a bone can be observed by grinding a slice 

 of bone extremely thin and mounting it on a slide for inspection under 

 a microscope. Bone contains not only bone-secreting cells, but also bone- 

 destroying cells. By the action of these two types of cells, the shape of a 

 bone may be altered to resist changing stresses and strains. Bone forma- 

 tion and destruction is regulated by the availability of calcium and 

 phosphate, by the presence of vitamin D and by the hormone secreted 

 by the parathyroid glands. The marrow cavity of the bone may contain 

 yellow marrow (largely a fat depot) or red marrow, the tissue in which 

 red and certain white blood cells are formed. 



Muscular Tissues. The movements of most animals result from the 

 contraction of elongated, cylindrical or spindle-shaped cells, each of 

 which contains many tiny, longitudinal, parallel, contractile fibers called 

 myofibrils. Muscle cells perform mechanical work by contracting— by 

 getting shorter and thicker; they are unable to do work by pushing. 

 Three types of muscle tissue are found in vertebrates: skeletal, cardiac 

 and smooth (Fig. 3.13). Cardiac muscle is found only in the walls of 

 the heart; smooth muscle in the walls of the digestive tract, the urinary 

 and genital tracts, and the walls of arteries and veins; and skeletal mus- 

 cle makes up the muscle masses which are attached to and move the 

 bones of the body. Cardiac and skeletal muscle cells are among the 

 exceptions to the rule that cells have but one nucleus; each of these 



