SPECIAL PHYSIOLOGY. 



sutures ; after this, they still continue to grow at their edges, until the cra- 

 nium has reached its full size an arrangement needed to suit the rapid 

 growth of the brain. The clavicle is likewise in part formed from membrane, 

 but afterwards in cartilage also. 



The mtra-cartilaginous mode of formation of bone occurs in the basal por- 

 tions of the cranium, viz., in the ethmoid, in nearly the whole of the sphenoid, 

 in the petrous and mastoid parts of the temporals, and in the occipital bone 

 below the occipital eminence. It also occurs in all the rest of the skeleton, 

 excepting only the clavicles. For each bone there is developed a separate pre- 

 cursory cartilage, which is inclosed in a definite perichondrium, and is at first 

 small and rudimentary in form, but gradually acquires, as it grows, the gen- 

 eral shape of the bone which is to be developed from it. Practically, therefore, 

 the skeleton is, at first, and for a long time, more or less cartilaginous. 



Certain centres of ossification, one, two, three, or even more, according to the 

 size and form of the future bone, appear at definite spots in the cartilage, and 

 extend into it as the latter increases in size. The cartilage continues to grow 

 in the direction of the articular surfaces of the joints, and also in that of the 

 various processes, until the development of the bone is complete. The bony 

 tissue also goes on growing in the same direction, by the successive formation 

 of the primary and secondary osseous tissue. But in other directions, and 

 especially towards the sides of the bones, the precursory cartilages, sooner or 

 later, cease to grow, and then the further increase in such directions is accom- 

 plished by intra-membranous ossification beneath the soft and growing peri- 

 osteum. 



Suppose in a long bone, for example, a single ossific centre to form in the 

 middle of the precursory cartilaginous shaft, as indeed is always the case. 

 Then, separate ossific centres subsequently appear at the ends, constituting 

 what are termed epipliyses (FT?/, epi, and 0vw phuo, I grow), and in the larger 

 bones, other smaller pieces are developed at the apices of the more remarkable 

 projections or processes. The precursory cartilage of the bone at last ceases 

 to grow in width, and, henceforth, the shaft of the growing bone is steadily 

 increased in diameter, by successive subperiosteal intra-membranous deposits 

 on its outer surface. At the same time, the medullary cavity is formed by a 

 continuous absorption going on within. A platinum wire placed around the 

 growing humerus or femur of a young pigeon, is found, after a time, inclosed 

 in the substance of the bone, or, if examined a little later, in the hollow of the 

 bone itself. But the precursory cartilage continues to grow in length long after, 

 and the bony shaft, and the epiphyses developed at the ends, ultimately meet, 

 but do not coalesce by osseous tissue until the full length of the bone has been 

 attained. This is evidently a provision for securing a progressive elonga- 

 tion of the bone during many years, together with a proper development of the 

 articular ends of the bones, all that time. At the ends of the bones, very thin 

 layers of the precursory cartilage remain permanently unossified, and form the 

 articular cartilages of the joints. Immediately beneath this articular cartilage 

 is a thin stratum of ossified cartilage or primary bone, recognizable by being 

 smoother and more compact than the rest. This is the only part of the pri- 

 mary bone which is permanent. The rest of this, and, indeed, the earliest 

 formed, and many succeeding portions of the secondary bone, and also the 

 subperiosteal mtra-menibranous bone, must be completely absorbed, before 

 any long bone has completed its growth ; for the young bone would easily lie 

 in what becomes, by continuous absorption, the medullary cavity of the full- 

 grown bone. The mode of increase in long bones is well shown by giving, at 

 stated intervals, to young pigs or other animals, madder mixed with the food. 

 The coloring matter of this root has an affinity for the salts of lime, and when 

 it is being taken in the food, the .bone then formed has a reddish tinge, whilst 

 the bone deposited at other times is yellowish-white. By this means it is 

 proved that successive additions are made at the surface and ends of the grow- 

 ing bone, and that absorption of the bone is continually taking place in its 

 interior. Again, the distance between two holes made, one above the other, 

 in a young bone, is not increased by its subsequent growth. (Hales, Hunter, 

 Duhamel) ; whereas a ring of wire placed closely around a growing bone is soon 



