188 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



representing tarsale 1, supports the calcar and 1st digit, the other 

 represents tarsale '2 + tarsale 3, and supports the 2nd and 3rd 

 digits. Tarsalia 4 and 5 are not distinctly represented, and there 

 is no centrale. The tarsus is succeeded by the metatarsus. The 

 base of the calcar is formed by a minute extra metatarsal, while 

 the slender melatarsals 1-5 belong to the corresponding digits. 

 A small flattened phalanx completes the calcar, and 2, 2, 3, 4, 3 

 slender phalanges terminate digits 1, 2, 3, 4, 5, respectively. 



Cartilage and bone may both be considered as modifications of 

 connective tissue in which the matrix is very plentiful. 



The most typical kind of cartilage is made up of a clear, homo- 

 geneous matrix, in which are numerous small cavities, lacunae, 

 connected with one another by fine channels, and containing 

 nucleated cartilage-cells. By the division of these, and the depo- 

 sition of fresh matrix, cartilage grows, especially near its surface, 

 which is covered by a connective-tissue membrane, the perichon- 

 drium, in which nerves, lymphatics, and blood-vessels run. 



The matrix of bone is densely fibrous connective tissue, impreg- 

 nated with lime salts, principally carbonate and phosphate. In 

 this are imbedded numerous much-branched bone-cells, which lie 

 in lacunae connected by fine tubules, canaliculi, into which the 

 cell-processes are continued. The matrix is traversed by blood- 

 vessels, contained in Haver sian canals. Bone may be either spongy, 

 as in the epiphyses of long bones and the interior of flat and short 

 bones, or compact, as seen in the shafts of long bones and the 

 exterior of flat and short bones. A membrane, the periosteum, 

 similar to perichondrium, covers the outer surface of bones. Its 

 inner part is made up of rounded cells, osteoblasts, which during 

 growth add fresh layers of bone to the outside. Ossification 

 always starts from definite centres. Membrane bones commence 

 as calcified networks of connective-tissue fibres covered by osteo- 

 blasts, while cartilage bones are pre-formed in solid cartilage, 

 covered by periosteum. Into this solid cartilage processes of the 

 osteoblastic part of the periosteum penetrate to form the marrow, 

 which first absorbs the cartilage, and then replaces it by spongy 

 bone. In the shafts of long bones this is absorbed in its turn to 

 form a continuous marrow-cavity. 



Cartilage bones can continue to lengthen by growth at their 

 ends. The new bone is formed in the way just described, from 

 the cartilage found there. 



