60 THE TISSUES. 



by the osteogenetic layer of the periosteum (subperiosteal bone-- 

 formation), which are substantially identical processes, con- 

 stitute the membranous method of bone-formation. All per- 

 manent bone is formed in this way. The process begins with 

 a deposition of osseous matter upon some of the fibres, a por- 

 tion of the connective-tissue cells assuming the function of 

 Gsteoblasts. A bony reticulum is thus formed, in the meshes 

 of which bone-formation is continued by the osteoblasts. 

 Along with the formation of bone, absorption, replacement, 

 and remodelling of the bone constantly go on. Osteoclasts 

 excavate spaces in the bone previously laid down, in which 

 new Haversian systems are formed. The remains of old 

 Haversian and peripheric lamella thus partially destroyed 

 constitute the interstitial lamellae. The spaces occupied by 

 osteoclasts are called Howship's lacunce. In the bones which 

 develop from membrane a membranous portion persists and 

 continues to grow between the ossifying areas or at the 

 margins, until the bones attain their full growth, when they 

 are completely ossified. 



Muscle. 



The muscle-cells are specialized cells having the property of 

 contractility highly developed. Their function is motor, to 

 effect the motions of the body. Muscle is derived from the 

 mesoblast. Muscular tissue is mainly made up of the cellular 

 elements, the intercellular substance being scanty and incon- 

 siderable. There are three varieties of muscle-cells : 



Striated, or voluntary ; 



Cardiac ; 



Involuntary, or non-striated. 



Striated or striped muscle (Figs. 27, 28) makes up all the 

 voluntary muscles of the body. The cells are very much 

 elongated protoplasmic fibers, generally 10 to 50 micromilli- 

 metres in diameter, and 20 to 40 millimetres long, or even 

 longer. In shape the cells or fibres are cylindrical or pris- 

 moidal, slightly fusiform, with tapering or blunt rounded 

 extremities ; in cross-section they are usually rather polygonal. 

 Each cell is inclosed in a delicate homogeneous separable 

 envelope, the sarcolemma. The fibres are usually straight and 



