112 GENERAL BIOLOGY 



and dense; the latter being composed of a comparatively loose arrange- 

 ment of plates and parts, lacks the strength of compact bone. The 

 spongy or cancellous type is found in the center of the vertebrae and to 

 a small extent within some of the long bones. Bones such as the femur 

 and, in fact, all of the long supporting bones in the body, must be rather 

 compact. A cross section of any of these long bones will show the outer 

 hard portion of a compact bone with an inner soft marrow and a thin 

 surface layer over the outside called the periosteum ( ), 



quite similar in structure to the perichondrium surrounding cartilage. 

 The arrangement of the layers in compact bone is concentric, and the 

 layers themselves are known as lamellae ( ). These 



lamellae contain numerous lacunae in which the bone-cells proper are 

 found. Fine branching tubes or canaliculi containing processes from 

 the bone-cells are given off from the lacunae and extend in all directions, 

 often anastomosing ( ) with the neighboring 



canaliculi. 



Bones grow like trees in that there are succeesive layers added to 

 the outside. The cells forming the inner layer of the periosteum, known 

 as osteoblasts ( ), are continually giving rise to new 



bone cells, which cause new layers of bony substances to be deposited 

 between the periosteum and the old bone. New layers may, however, 

 be added on the inner surface between the walls and the marrow cavity. 



Muscle tissue (Fig. 41) is composed of elongated cells or fibers 

 united by connective tissue, as already mentioned. There are three 

 types, the voluntary or striated, the involuntary or unstriated, and the 

 automatic or branched, a sort of combination of these two known as 

 heart-muscle. 



The nonstriated fibers are rather simple in structure, commonly 

 spindle-shaped with a single nucleus near the center, often elongated. 

 The ends of the fibers may be branched, but are not usually so. The 

 length of the fibers varies to a considerable extent. They may be very 

 narrow, or short and comparatively thick. In the involuntary muscle 

 fibers there is usually no cross striation, but one may find delicate longi- 

 tudinal strands called fibrillae, usually considered to be the contractile 

 elements of the cell. The cell wall itself is thin and transparent. Non- 

 striated muscles respond to stimuli quite slowly, being also somewhat 

 slow to relax after the function has been performed. It is found particu- 

 larly in those branches of the body where sudden movement is not re- 

 quired, such as in the muscular coats of the alimentary canal, in the 

 walls of blood vessels, in various ducts, in the lungs, urinary and gall 

 bladders, and around glands in the skin, and also in the iris and ciliary 

 muscle of the eye. 



Striated muscle fibers are more complicated in structure. They 

 possess several spindle-shaped nuclei scattered throughout the cell, each 

 nucleus surrounded by a small amount of undivided cytoplasm ( ). 



