THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 641 



the perichondrium vascular, richly cellular connective-tissue processes 

 grow into the cartilage, dissolve its matrix, and unite with one 

 another in its centre. There arises a network of medullary [marrow] 

 cavities, in the vicinity of which there is a deposit of salts of lime (a 

 provisional calcification). The medullary spaces extend farther and 

 farther by destruction of the cartilaginous substance. Then there 

 are secreted by the superficially located medullary cells bone-lamellae, 

 which gradually increase in thickness. The osseous ' nucleus thus 

 formed slowly increases in ■ size, until finally the cartilage is almost 

 entirely replaced, only a thin layer of it remaining at the surface as 

 a covering to the bone. 



The ossification of the wrist- and ankle-bones is therefore purely 

 endochondral, and proceeds ordinarily from one, sometimes from two, 

 centres of ossification. It does not begin until very late — in the first 

 year after birth. The only exception occurs in the foot, where the 

 OS calcis and astragalus acquire a bony nucleus in the sixth and 

 seventh months, and the cuboid begins to ossify a short time before 

 birth. In the others ossification takes place after birth, and, as 

 KoLLiKER states, in the following order : — 



I. In the hand. (1) Os magnum and unciform (first year) ; 

 (2) cuneiform (third year) ; (3) trapezium and lunar (fifth year) ; 

 (4) scaphoid and trapezoid (sixth to eighth year) ; (5) pisiform 

 (twelfth year). 



II. In the foot. (1) Os scaphoideum (first year) ; (2) internal and 

 middle cuneiform (third year) ; (3) external cuneiform (fourth year). 



Concerning the cartilaginous fundaments of a special centrale carpi, wliioh 

 usually is not retained as a separate carpal element (Rosenberg), as well as 

 a special intermedium tatsi or trigonum (Babdeleben), the text-books of 

 compaiative anatomy are to be consulted. 



The process of ossification is more complicated in the long car- 

 tilages, in which, moreover, it begins much earlier, usually even in 

 the third month of embryonic life. The course of ossification is 

 fairly typical. 



At first a perichondral ossification takes place midway between 

 the ends of each cartilage in the humerus and femur, tibia and 

 fibula, radius and ulna. From the perichondrium there is deposited 

 upon the already formed cartilage bony tissue instead of a car- 

 tilaginous matrix, so that the middle portion of the cartilage becomes 

 ensheathed in a bony cylinder, which is continually increasing in 

 thickness. 



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