THE HISTOGENESIS OF MUSCLE 29I 



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

 osteoblasts 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 changed to bone. In adults, the periosteum is regarded as especially important 

 in the regeneration of bone tissue. Macewen (1912), however, rejects this view. ' 



Joints. — In joints of the synarthrosis type in which Kttle 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 mesen- 

 ch3Tna between the bones develops into an open connective tissue in which a cleft 

 appears, the joint cavity. The cells Kning this cavity flatten out and form a more 

 or less continuous layer of epithelium, the synovial membrane. From the con- 

 nective tissue surrounding the joint cavity are developed the various fibrous 

 ligaments t3^ical of the different joints. Ligaments or tendons apparently cours- 

 ing through the adult joint cavities represent secondary invasions, which are 

 covered with reflexed synovial membrane, and hence are really external to the 

 cavity. 



THE HISTOGENESIS OF MUSCLE 



The muscular system is composed of muscle fibers which form a tissue in 

 which contractility has become the predominating function. The fibers are of 

 three types : (1) 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 movements; (3) striated cardiac muscle, form- 

 ing the myocardium of the heart. All three types are derived from the meso- 

 derm. 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 in general may be said to arise from the mesench)ane, or 

 from embryonal connective tissue. Its development has been studied by McGill 

 (Internat. Monatschr. f. Anat. u. Physiol., vol. 24, 1907) in the esophagus of 

 pig embryos. The stellate cells of the mesenchyma enlarge, elongate, 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-contractile myogUa fibrils (Fig. 297) similar 

 to the primitive fibrillae of connective tissue. The myogha fibrils may extend 

 from cell to cell, thus connecting them. These fibrils are the products of coa- 

 lesced granules found within the cytoplasm of the myoblasts. In embryos 

 of 30 mm. fine myofibrillae are differentiated in the cytoplasm of the myoblasts 



