Histology of the Frog 113 



of two types of bone, namely, compact, and spongy or cancellous. The 

 former is firm and dense while the latter is composed of a comparatively 

 loose arrangement of plates and parts, thus lacking the strength of com- 

 pact 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 perios- 

 teum ( ). This latter is 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 successive layers are 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, however, may 

 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 the first 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 

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

 walls of blood vessels, in various ducts, in the lungs, in the 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 than non- 

 striated muscles. They possess several spindle-shaped nuclei scattered 

 throughout the cell, each nucleus surrounded by a small amount of 



