208 HISTOGENESIS 



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 

 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 especially important in the regener- 

 ation of bone tissue. 



The special development of the various bones of the skeleton is beyond the scope of this 

 book. The student is referred to the various text-books of anatomy, to Kollmann's Handatlas 

 of Embryology, vol. i, and to Bardeen's chapter in Keibel and Mall (vol. i, p. 316 ff). 



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 mesen- 

 chyma 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 synovial membrane. From the con- 

 nective tissue surrounding the joint cavity are developed the various fibrous 

 ligaments typical of the different joints. 



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 : (i) smooth muscle cells found principally in the walls of the viscera and 

 blood-vessels; (2) striated cardiac muscle, forming the myocardium of the heart; 

 (3) striated voluntary muscle, chiefly attached to the elements of the skeleton and 

 producing voluntary movements. 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 mesenchyme, or 

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

 (Internat. Monatschr. f. Anat. u. Physiol., vol. 24, pp. 209-245, 1907) in the 

 esophagus of pig embryos. The stellate cells of the mesenchyma enlarge, elongate 



