62 CARTILAGE AND BONE 



round the canals, or parallel to the bone's outer surface. By 

 decalcifying bone, fibres Avere discovered by Sharpey to extend 

 in varying quantity through the matrix of ossein. Just as the 

 cartilages are surrounded by a connective -tissue covering, the 

 ' perichondrium,' so the bones are enclosed in a connective-tissue 

 ' periosteum,' from which blood-vessels and nerves penetrate to the 

 canals. 



Such is the structure of typical bone in mammals, whether it 

 be preceded by cartilage or formed directly in connective - tissue 

 membranes. But the finer structure of bone may vary con- 

 siderably. The Haversian canals, serving chiefly for nutrition, 

 are found to be less numerous, or even entirely absent, in the 

 bones of very small animals, and in structures of small size. The 

 bones in the lower classes of vertebrates are often of less regular 

 and elaborate formation. The lamellae may be scarcely indicated, 

 the bone-cells scattered, and the canals in the shape of irregular 

 spaces. Kolliker [270] has shown that, in the Teleostei, the bone 

 may in some cases be poor in matrix and very rich in fibres (Xiphias, 

 Gadus); in others the bone -cells may be exceedingly rare, or 

 practically absent in the adult tissue (Pleuronectes) ; again, it 

 may acquire a structure similar to that of dentine (Stewart, 

 Fistularia [425]). Nevertheless, true bone is undoubtedly 

 developed in all the classes of the Craniata, from the fish upwards, 

 though not in all Craniates. 



The question of the origin and growth of cartilage and bone 

 during individual development is important. The history of 

 cartilage in ontogeny is simple enough : undifferentiated cells, in 

 certain regions of the connective - tissue system, secrete a clear 

 matrix round themselves, divide, and still secrete until a mass of 

 cartilage is formed surrounded by the perichondrium. Such a 

 cartilaginous skeleton can continue to grow in all its parts, either 

 retaining or altering its shape according to the needs of the animal. 

 This account, however, is based on comparatively modern investi- 

 gation. Before the significance of the cells in cartilage was 

 understood, it was for a long time held that cartilage is formed 

 from a fluid blastema by a process of gradual solidification. This 

 process, carried a step farther, was supposed to give rise to bone 

 Avith the aid of calcification. 



Such a crude theory had to be abandoned when the importance 

 of the cells came to be appreciated. But it Avas still supposed, until 

 the middle of last centiuy, that cartilage becomes actually con- 

 verted into bone by the solidification and calcification of the 

 existing matrix, and by the modification of the enclosed cells 

 into branching bone-corpuscles. This A'ieAv received at first the 

 support of Schwann, Tomes, Virchow, Kolliker, Banvier, and 

 others, and appears to be still held in a more or less modified form 



