MUSCULAR TISSUES 25 



referred to. In only a few regions, as already indicated, the car- 

 tilage persists throughout life. The difference between the two 

 types of bone is not fundamental, however, since both are formed 

 by soft connective tissue, the latter invading a cartilage model 

 when this exists and destroying it bit by bit previous to the de- 

 position of bone-matrix. 



Membrane bone occurs less extensively than replacing bone. It 

 is exemplified by the roofing and facial bones of the skull, most 

 of which have a flattened tabulate form, and by the clavicle. It 

 is formed in connective-tissue membranes and may contain cartilage, 

 but does not develop on a cartilage basis. Skeletal units of this 

 type lie superficial to the other skeletal elements, a feature which 

 is due to the fact that they represent surface plates which in lower 

 vertebrates are associated with the skin. 



Only in a few cases are the bones of the skeleton solid — as a 

 rule they consist of a fairly thin shell of hard or compact bone 

 surrounding a central mass of spongy or cancellous bone. This 

 arrangement is one of great mechanical strength, combined with 

 lightness, and at the same time provision is made in the interior 

 of the bone for blood-vessels and marrow-spaces. Thus in a long 

 bone (Fig. 12) the central portion or shaft consists of a cylinder of 

 compact bone surrounding an extensive space, the marrow-cavity, 

 filled with soft vascular tissue which, in the adult animal, is the 

 most important region of red blood formation. The ends or 

 extremities consist each of a thin shell of compact bone continuing 

 that of the shaft and surrounding a mass of cancellous tissue. In 

 the short, flat, or irregular bones of the skeleton no continuous 

 marrow-cavity is formed. 



It may be noted that, in addition to its function as a mechani- 

 cally supporting or protecting material, bone serves as a reservoir 

 •for the important substance, calcium, which can be dissolved in 

 the blood or redeposited in the bone as the needs of other parts of 

 the organism may demand. 



3. Muscular Tissues 



Muscular tissues are the active portions of the individual 

 muscles which move the skeleton and of the muscle coats of visceral 



