86 PRACTICAL HISTOLOGY. 



(L.) Examine cancelli, articular cartilage, and a layer 

 of calcified cartilage close to the bone. 



(H.) a. Osteoblasts. In the cancelli osteoblasts will 

 be found close to the bone. Observe their irregular shape, 

 some with distinct processes, and all of them having a bright 

 refractive appearance. The other cells found in marrow 

 will be examined in a preparation from fresh bone. 



b. Articular cartilage. Near the articular surface the 

 cells are flattened ; about the middle of the cartilage they 

 are in irregular groups ; near, and in the calcified layer, 

 they are arranged in rows, owing to repeated proliferation 

 of the cartilage cell by cleavage at right angles to the long 

 axis of the row. The lime has of course been removed 

 from the matrix in the calcified zone, but in specimens not 

 fully decalcified, a fibrillation of the matrix similar to that 

 seen in the rib ( 1 1 6) may be recognised. On very care- 

 fully examining the line of junction between the cartilage 

 and the bone, one can here and there see a cartilage pro- 

 toplast becoming surrounded by the matrix of the bone 

 that grows up around it. The production of a bone cell 

 from a cartilage cell may be thus demonstrated ; but this 

 mode of bone formation seen above the epiphysis in an 

 adult animal must be regarded as exceptional, and differing 

 decidedly from the mode discernible under the epiphysis in 

 a young animal. 



126. The intra-cartilaginous development of bone may 

 be very clearly studied in a V. S. of a long bone or scapula 

 of a kitten at birth, and in a V. S. of the phalanx of a 

 human foetus at the fourth month (doubtless the phalanx 

 of a kitten would show the same, but this we happen not 

 to have examined). The bones should be softened in 

 picric acid, and the former may be unstained while the 

 latter is stained with picro-carmine. It is advantageous to 

 inject the blood-vessels with soluble Prussian blue ( 333) 

 previous to softening. 



127. The vertical section of the phalanx, prepared as 

 above, is exceedingly instructive. 



(L.) The ossification begins in the middle of the shaft, 

 and then extends towards the ends. A wedge-shaped piece 



