1880.] Ossification of the Terminal Phalanges of the Digits. 65 



the changes that are taking place within ; the whole process resulting 

 in the assumption by the bony cylinder of the form of a dice-box with 

 both ends open, and the median constriction partially or entirely filled 

 in from the outside. Subject to its peculiar conformation, the growth 

 of the bony layer in the ungual phalanx is analogous to the foregoing 

 process. A thickening takes place by the addition of new bone to the 

 outer surface of the cap, and simultaneously with this its edges grow 

 backwards along the cartilage from the tip towards the base (in the 

 same direction as that previously taken by the process of cartilage 

 calcification), so that the cap becomes deeper and deeper, and finally 

 reaches a stage in which it may be compared to a thimble fitting over 

 the cartilaginous phalanx and enclosing it almost up to its base (see 

 figs. 1, 2, 3, 4, p, p) . The deposition of new bone along the outer surface 

 of this cap or thimble may take place relatively much more quickly at 

 the summit than at the sides, so that an expansion is formed, which 

 may be of considerable size, and which no doubt bears a relation to the 

 -conformation of the future nail, hoof, or claw (see fig. 2). The next 

 stage in the development of the bone is marked by the irruption of the 

 subperiosteal tissue with blood-vessels and osteoblasts into the shaft at 

 or near the point where the cartilage first began to calcify. This point 

 in all other cases will be found about the middle of the shaft, but in 

 the ungual phalanx, as has been seen, at the tip. In fig. 1, which 

 represents a section in the sagittal plane through the growing ungual 

 phalanx of a foetal cat, the irruption is seen to have taken place at a 

 point just below the tip on its plantar aspect, and this would appear to 

 be the most usual locality. In figs. 2, 3, representing similar sections 

 from the pig and the human subject at an earlier stage of development, 

 the points at which the invasion will probably begin are indicated by 

 the letters i, i. The advance of the osteoblastic tissue into the carti- 

 laginous diaphysis, its gradual absorption of the primary bone, and the 

 laying down of true osseous tissue in its stead, follow in all cases the 

 t direction already taken by the previous process of calcification ; that is 

 to say, in other long bones from the middle towards the two ex- 

 tremities, in the ungual phalanx from the tip towards the base. Thus, 

 to sum up, in the development of the ungual phalanx the three pro- 

 cesses of cartilage calcification, growth of the subperiosteal intramem- 

 branous bone, and deposition of true bone in the shaft along the line of 

 advance of the osteoblastic ingrowth, take the distal extremity of the 

 shaft instead of its middle for their starting-point, and proceed in one 

 uniform direction from tip to base, instead of advancing in two 

 contrary directions at the same time. Hence it seems that the distal 

 extremity of the ungual phalanx corresponds morphologically with the 

 centre of the diaphysis in other long bones. 



At a period of growth subsequent to the complete ossification of the 

 diaphysis, an epiphysis forms, as is well known, in the cartilaginous 



