2QO HISTOGENESIS 



in length is completed, the epiphyses, by the ossification of the intervening 

 cartilage, are united to the diaphysis. 



The shaft of the long bones grows in diameter by the peripheral 

 deposition of bone lamellae and the central resorption of the bone. In 

 the larger long bones, spongy, or cancellated bone tissue persists at the 

 ends, but in the middle portion a large medullary, or marrow cavity is 

 developed. This is filled chiefly with fat cells and constitutes the yellow 

 bone marrow. 



Regeneration of Bone. If bone is injured or fractured, new bone is developed by osteo- 

 blasts derived either from the periosteum or from the bone marrow. The repair of a 

 fracture is usually preceded by the formation of cartilage which unites the ends of the bones 

 and is later replaced by bone. In adults, the periosteum is regarded as especially important 

 in the regeneration of bone tissue. 



Joints. In joints of the synarthrosis type, in which little movement 

 is allowed, the mesenchyma between the ends of the bones differentiates 

 into connective tissue or cartilage. This persists in the adult. 



In joints of the diarthrosis type, the bones are freely movable. The 

 mesenchyma between the bones develops into an open connective tissue 

 in which a cleft appears, the joint cavity. The cells lining this cavity 

 flatten out and form a more or less continuous layer of epithelium, the 

 synomal membrane. From the connective tissue surrounding the joint 

 cavity are developed the various fibrous ligaments typical of the different 

 joints. Ligaments or tendons which apparently course through the adult 

 joint cavities represent secondary invasions, covered with reflexed synovial 

 membrane and hence really external to the cavity. 



THE HISTOGENESIS OF MUSCLE 



The muscular system is composed of muscle fibers ; these form a tissue 

 in which contractility has become the predominating function. The 

 fibers are of three types: (i) smooth muscle cells, found principally in the 

 walls of the viscera and blood vessels; (2) striated skeletal muscle, chiefly 

 attached to the elements of the skeleton and producing voluntary move- 

 ments; (3) striated cardiac muscle, forming the myocardium of the heart. 

 All three types are derived from the mesoderm. The only exceptions are 

 the smooth muscle of the iris, and the smooth muscle of the sweat glands, 

 which are derived from the ectoderm. 



Smooth Muscle. Certain stellate cells of the mesencyhma enlarge, 

 jslongate, and their cytoplasm becomes more abundant. The resulting 

 spindle-shaped cells remain attached to each other by cytoplasmic bridges 

 and develop in the superficial layer of their cytoplasm coarse, non-con- 

 tractile myoglia fibrils (Fig. 297), similar to the primitive fibrillae of con- 



