1880.] Ossification of the Terminal Phalanges of the Digits. 69 



arrangement characteristic of approaching calcification ; at cc the calcifi- 

 cation is complete, a marks the furthest point yet reached by the 

 osteoblastic invasion, where secondary areolae containing numerous 

 osteoblasts have been formed by the partial demolition of the calcified 

 cartilage ; and at % the ingrowth of the subperiosteal tissue with blood- 

 vessels and osteoblasts is seen to have begun close to the tip on its 

 plantar aspect, and to be spreading back towards the base of the 

 phalanx, having effected the total absorption of the primary bone in the 

 neighbourhood of the point of invasion ; p, the subperiosteal cap of 

 intramembranous bone, at this stage thin and incomplete ; osteogenic 

 fibres and osteoblasts are seen along its outer border, and lacunae in its 

 substance, pp, layer of bone marking the former limit of the sub- 

 periosteal cap, which has suffered to a great extent from absorption, and 

 is being remodelled on a larger scale, ft, et, flexor and extensor tendons, 

 some fibrous bundles from which are seen to be prolonged into the 

 growing bone, bv, blood-vessels in the osteoblastic tissue. 

 [Magnified about 85 diameters.] 



Figure 2. Sagittal section through the terminal phlanax of the fifth digit of the right 

 manus of a human foetus about the fourth month. Decalcified in picric 

 acid, stained with magenta, c, unaltered cartilage; cc, calcified cartilage. 

 At n, a region occurs where the cartilage presents unusual character- 

 istics, the cell-spaces being smaller and the matrix more deeply stained 

 than in the areas on either side, which show the more typical appear- 

 ance of primai-y bone, p, the subperiosteal cap of intramembranous 

 bone, with osteogenic fibres and osteoblasts. Many fibrous bundles, con- 

 tinuous with the tendons et and ft, are seen as in the last figure to be 

 prolonged into the bone. The ungual expansion at the summit of the cap 

 consists of two masses (d and pa), separated by a notch, i, which may 

 represent the point of future irruption. A series of sections from the same 

 digit shows that what appears here as a notch is really a pit sunk in the 

 middle of the ungual expansion. Of the two masses (d ancl^a), the one 

 towards the dorsal surface (d) is characterised by possessing in proportion 

 far fewer lacunae with their included bone-cells than the protuberance (pa) 

 on the palmar side. This distinction applies also to the dorsal and palmar 

 sides of the cap as far as they extend. In this specimen no irruption 

 lias yet taken place, and the cartilage preserves its original form. The 

 conical contour indicated by the figure is due to the fact that expansion 

 can only take place in cartilage as yet untouched by the process of 

 ossification, its growth when this begins being checked either by the 

 calcification of its own matrix or by the obstacle opposed to it by the 

 investing subperiosteal layer of bone. Hence, only the proximal area of 

 the cartilaginous phalanx is capable of lateral expansion, and since the 

 growth of the cartilage continues for a long time after the beginning of 

 the process of ossification at its tip, the conical shape seen in this and 

 other instances results. The same cause produces the well-known hour- 

 glass form of the cartilage seen during the growth of other long bones. 

 [Magnified 65 diameters.] 



Figure 3. Sagittal section through the terminal phalanx of one of the middle digits 

 of the manus of a foetal pig 6 centims. long. Decalcified in picric acid? 

 stained with magenta, c, unaltered cartilage ; cc, calcified cartilage, p, 

 subperiosteal cap of bone, which is greatly thickened on the palmar 

 aspect of the phalanx. A deep cleft is seen separating the greater bulk 

 VOL. XXXI. G 



