DEVELOPMENT OF THE OSSEOUS SYSTEM. 989 



.In the first row the astragalus corresponds to the tibia, the calcaneum to the fibula. 

 As a third bone of the first row there is particularly worthy of note a small piece 

 of bone adherent to the astragalus at the insertion of the posterior fibular liga- 

 ment of the tarsus. This corresponds to the semilunar bone of the carpus, 

 is indicated in the second month as an independent cartilage, and appears in 

 urodela and marsupials as a typical intermediate tarsal bone, although it does not 

 undergo development in man. 



In the second row (as in the carpus) the rudiments of the fourth and fifth 

 bones are united as the cuboid. The tarsal bones ossify in the following order: 

 Calcaneum (beginning of the seventh month) : astragalus (beginning of the eighth 

 month) ; cuboid (end of the tenth month) ; scaphoid or central (between the 

 first and fifth years) ; first and second cuneiform (third year) ; third cuneiform 

 (fourth year) . In the heel of the calcaneum an accessory center develops between 

 the fifth and the tenth year, and unites after puberty. 



The metatarsal bones are formed in the same way as the metacarpal bones, 

 but later. 



After numerous measurements of the diaphyses of long bones in embryos 

 and fetuses, the writer has been able to establish the following general principles: 

 i. Up to the ninth or tenth week the ossified middle portions of the long bones in 

 the upper part of the body are the largest, and in the following order: Inferior max- 

 illa, clavicle, humerus, radius, ulna, femur, tibia, fibula. 2. From the sixth month 

 they range in size as they do in the adult. 3. The diaphyses of the tubular bones 

 of the upper extremity are at all periods of fetal life relatively larger than those 

 of the lower extremity. 4. In the first half of fetal life, the diaphyseal bones 

 grow in the same length of time much more than they do later; even twice as 

 much and more. Of the epiphyses, ossification takes place earliest in those that 

 have the greatest relation in weight as compared with their diaphyses. 



In the formation of bone from cartilage, the cartilage-cells multiply in the 

 dilating spaces in which they are contained. The latter unite to form large 

 cavities, upon whose walls the new bone-mass is deposited in layers. Whether, 

 under such circumstances, the descendants of the cartilage-cells, greatly increased 

 in number by division, become transformed into bone-cells, or whether the cells 

 utilized for this purpose grow, together with the blood-vessels, into the ossifying 

 cartilage (while the cartilage-cells degenerate) is still an open question. 



Dried bone consists one-third of organic matrix (bone-cartilage, by boiling 

 reduced to gelatin), further of neutral calcitftn phosphate (57 per cent.), calcium 

 carbonate (7 per cent.), magnesium phosphate (from i to 2 per cent.), calcium 

 fluorid (i per cent.) and a trace of chlorin. Fresh bone contains about 23 per 

 cent, of water; the marrow, fluid bone-fat, albumin, hypoxanthin, cholesterin, 

 and extractives. Red marrow contains more iron in conformity with the hemo- 

 globin present. 



The bones (for example, the tubular bones) grow in thickness by deposition 

 from the periosteum, the cells of the latter being transformed as osteoblasts 

 into bone-corpuscles. 



In part the peripheral regions (parietal layer) of the osteoblasts, resembling 

 epithelium lying close together, are transformed into the hardened matrix of the 

 bone, the cells becoming star-shaped bone-corpuscles. In part, however, isolated 

 star-shaped periosteal cells also are transformed into bone-cells, a hardening 

 blastema being poured out between them and taking up the fibers of the perios- 

 teum as Sharpey's fibers into the substance of the bone. Coincident with the 

 growth of the bone at its periphery, the medullary cavity becomes larger through 

 absorption. Rings placed around the shaft of long bones in young animals subse- 

 quently come to be within the medullary cavity. 



The growth of the bones in length takes place in such a manner that the strip 

 of epiphyseal cartilage adjacent to the diaphysis undergoes constant ossification, 

 while new cartilage is being constantly produced at the peripheral extremity. 

 When the growth of the bone is completed, the epiphyseal cartilage finally ossifies 

 as a whole. Whether in addition to this growth of the bone through apposition, 

 there is growth also through intussusception or interstitial expansion, investiga- 

 tion (whether two pegs driven into the shaft of a growing bone become further 

 removed or not) has not yet determined conclusively. 



The form into which the bones develop depends also upon external influences. 

 They develop the more rapidly the greater the activity of the muscles attached 

 to them. In overcoming pressure normally applied to a bone it yields in the 

 direction of least resistance, and becomes thicker in this direction. The bone 



