56 



THE ESSENTIALS OF HISTOLOGY 



numerous blood-vessels which have grown in at the same time. The 

 spaces are termed medullary spaces, and this second stage may be 

 termed the stage of irruption. 



FIG. 65. SECTION OF PART OF 

 ONE OF THE LIMB-BONES OF 

 A FCETAL CAT, AT A MORE 

 ADVANCED STAGE OF OSSIFI- 

 CATION THAN IS RkPKE- 

 SENTED IN FIG. 64, AND 

 SOMEWHAT MORE HIGHLY 

 MAGNIFIED. 



The calcification of the cartilage- 

 matrix has advanced from the 

 centre, and is extending between 

 the groups of cartilage-ceils 

 which are arranged in character- 

 istic rows. The subperiosteal 

 bony deposit (im) has extended 

 pan passu with the calcification 

 of the cartilage-matrix. The 

 cartilage-cells in the primary 

 areolae are mostly shrunken and 

 stellate ; in some cases they have 

 dropped out of the space. At ir 

 and in two other places an irrup- 

 tion of the subperiosteal tissue, 

 composed of ramified cells with 

 osteoblasts and growing blood- 

 vessels, has penetrated the sub- 

 periosteal bony crust, and has 

 begun to excavate the secondary 

 areolaa or medullary spaces ; p, 

 fibrous layer of the periosteum ; 

 o, layer of osteoblasts, some of 

 them are embedded in the osseous 

 layer as bone-corpuscles in la- 

 cunae ; U, blood-vessels occupied 

 by blood-corpuscles. Beyond the 

 line of ossific advance the perios- 

 teum may be noticed to be dis- 

 tinctly incurved. This incurva- 

 tion is gradually moved on, the 

 cartilage expanding behind it 

 until the head of the bone is 

 reached, when it forms the peri- 

 osteal notch or groove represented 

 in fig. 66, p. 57. 



In the third stage of endochondral ossification there is a gradual 

 advance of the ossification towards the extremities of the cartilage, 

 and at the same time a gradual deposition of fresh bony lamellae and 

 spicules on the walls of the medullary spaces, and on the surface of 

 the new bone under the periosteum. The advance into the cartilage 

 always takes place by a repetition of the same changes, the cartilage- 

 cells first enlarging and becoming arranged in rows, the matrix 

 between the rows becoming calcified, and then the calcified cartilage 

 becoming excavated from behind by the osteoblastic tissue so as to form 

 new medullary spaces (fig. 67). The walls of these are at first formed 



