66 



THE ESSENTIALS OF HISTOLOGY. 



tilage (fig. 74, ir). This is freely absorbed before it (fig. 75), so that 

 large spaces are produced which are filled with osteoblasts and contain 

 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. 



irn 



FIG. 74. SECTION OF PART OF 



ONE OF THE LIMB-BONES OF 

 A FCETAL CAT, AT A MORE 

 ADVANCED STAGE OF OSSIFI- 

 CATION THAN IS REPRESENTED 

 IN FIG. 73, AND SOMEWHAT 

 MORE HIGHLY MAGNIFIED. 



(From a drawing by J. Law- 

 rence. ) 



The calcification of the cartilage- 

 matrix has advanced from the 

 centre, and is extending between 

 the groups of cartilage-cells which 

 are arranged in characteristic 

 rows. The subperiosteal bony 

 deposit (im) has extended pari 

 passu with the calcification of the 

 cartilage-matrix. The cartilage- 

 cells in the primary areolse are 

 mostly shrunken and stellate ; in 

 some cases they have dropped out 

 of the space. At ir and in two 

 other places an irruption of the 

 subperiosteal tissue, composed of 

 ramified cells with osteoblasts 

 and growing blood-vessels, has 

 penetrated the subperiosteal bony 

 crust, and has begun to excavate 

 the secondary areolje or medullary 

 spaces ; p, fibrous layer of the 

 periosteum ; o, layer of osteo- 

 blasts, some of them are embedded 

 in the osseous layer as bone- 

 corpuscles in lacunae ; bl, blood- 

 vessels occupied by blood-corpus- 

 cles. Beyond the line of ossific 

 advance the periosteum may be 

 noticed to be distinctly incurved. 

 This incurvation is gradually 

 moved on, the cartilage expand- 

 ing behind it until the head of the 

 bone is reached, when it forms 

 the perioateal notch or groove 

 represented in fig. 77, p. 69. 



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 



