PROGRESS OF MICROSCOPICAL SCIENCE. 
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and on the exterior of bones. Internally, they exist in ossifying 
cartilage, just beyond the bone, where ultimately cancelli are formed, 
as in the bodies of the vertebrae and the ends of the long bones. 
They also are present on the walls of the larger medullary cavities in 
process of development ; as in the diploe of the cranial bones, in the 
medullary canals of long bones ; and the development of the frontal, 
sphenoidal, and other sinuses of the head and face, is intimately con- 
nected with their presence in these parts. On the external parts of 
bones they exist in the following situations : the dental grooves of the 
embryonic jaws ; many parts of the bones limiting the cranial cavity, 
both at the base and at the sides and summit ; on the walls of the 
vertebral canal and on the arches and bodies of the vertebra) ; on 
the walls of the orbit and of the nasal fossas ; on the coronoid and 
condyloid processes of the lower jaw, and on the zygomatic process of 
the upper jaw ; in the situations of the foramina and canals by which 
many of the bones are perforated, such as the foramen ovale, the 
foramen opticum, &c. ; and on certain parts of the bones of the trunk 
and extremities, especially in the neighbourhood of the articular ends 
of the bones ; as well as on the surface of the temporary teeth, where 
they are abundantly developed while these are undergoing absorption. 
Having described the situation where these osteoclasts are found, 
Kolliker proceeds to comment on their signification. Hitherto, he 
says, the normal resorption of bone has been attributed either to the 
chemical action of the fluids or of the medulla, or to some not very 
well-defined minute process. Thus Virchow supposes the osseous 
substance to pass into simple marrow-cells ; Bredichin and Rindfleisch 
regard the “ myeloplaxes ” or osteoclasts as bone-cells set free by 
removal of the earthy matter ; and Billroth believes that, in cases of 
disease, the bone is removed by the pressure of granulation-cells. 
Kolliker’s experiments have led him to the conclusion that tho 
resorption of osseous and dental tissue does not consist in a mere 
transformation of its cellular and other elements, but in a removal of 
them ; and that the multinucleated cells or osteoclasts are in no way 
formed from pre-existent osseous or dental tissue. The proofs in 
support of this view are the following : — 1. A bone or tooth, at the 
parts where it is studded with osteoclasts, shows no sign of transforma- 
tion of its cells, such as enlargement, multiplication of nuclei, &c. ; 
and it is extremely common to find, on the walls of the lacunae which 
contain the osteoclasts, partial erosions in the tissue, which in other 
respects presents no change. 2. The osteoclasts can always be readily 
separated from the osseous tissue ; there is no intimate connection or 
gradual transition between them. 3. After the absorption of the 
bone, small splinters not unfrequently remain ; these contain no cells, 
and cannot be transformed into cells. 4. The occurrence of osteo- 
clasts on the surface of the ivory of the milk-teeth shows that no 
elements of the dental tissue that is undergoing absorption can enter 
into their formation. 5. A most striking fact is the presence of 
osteoclasts on the surface of ivory pegs that have been driven into 
bone. Billroth has found that these pegs become eroded ; and 
Kolliker has found them to present on their surface not only lacunas, 
