66 



Mr. F. A. Dixev. 



t/ 



[NOV. 2 5 r 



head which, still remains at the proximal end of the phalanx, and this 

 becomes united to the shaft in the usual manner. 



The process of ossification in the terminal phalanx having now been 

 described in general terms, it only remains to notice certain special 

 points of interest that have come under observation during the investi- 

 gation of the subject. 



It should be first mentioned that the peculiarity described appears 

 to be universal, the same general description applying to any terminal 

 phalanx, whether taken from the manus or pes, and whether belonging 

 to a fully functional or to a mere aborted digit, such as the second 

 and fifth in the manus of the pig. An examination of the growing 

 bone with a view to this point would probably suffice to decide whether 

 the phalanx missing from certain digits in the manus of Pteropus and 

 other bats is really the third (as described, see Flower, " Osteology 

 of the Mammalia," 1876, p. 264), or whether it may not rather be the 

 second. 



A comparison of a series of specimens taken from man, the mole, 

 the rat, the pig, and other animals, seems to indicate that the primitive 

 cartilaginous terminal phalanx exists normally as a subcylindrical bar 

 of comparatively simple conformation ;* while the remarkable modifi- 

 cations of shape which the adult phalanx assumes throughout the 

 whole range of the mammalia are due to the superstructure erected on 

 this basis by purely intramembranous ossification in the form of the- 

 subperiosteal cap, and of what may be called the ungual expansion at 

 its summit. Thus, in the mole, the primitive cartilaginous terminal 

 phalanx presents no distinctive character ; it resembles in shape the 

 cartilage of the young pig, as seen in fig. 3, but is relatively a little 

 shorter and thicker. When, however, the subperiosteal growth begins, 

 it is seen that a separate centre for the accumulation of the bony 

 deposit has established itself on each side of the tip of the phalanx,, 

 the two being connected by a film of bone over the tip itself, so that 

 the resulting cap has two summits instead of one. At a later stage 

 these two summits are found to have increased considerably in height 

 and thickness, and to have encroached upon and partly filled up the 

 interval between them. In this way the growth of the bifid cap pro- 

 ceeds until, after the normal changes, the ungual phalanx of the manus 

 in the adult mole presents a highly modified appearance, being un- 

 usually long and deeply cleft at its extremity, thus contrasting strongly 

 with the cartilaginous basework on which it was fashioned. Again, 

 in the cat and other carnivora, the hood-like expansion of bone which 

 in the adult is reflected over the base of the claw must owe its origin 

 entirely to a process of intramembranous ossification, since no trace 



* This shape is liable to a slight modification from the growth of the cartilage at 

 its proximal end after the process of calcification has already begun at the tip. See- 

 description of fig. 2. 



