cvi BONE. 



and forms the lacuna, the canaliculi and their communications may be merely channels 

 left as vacuities in the osseous deposit, into which the processes of the now stellate 

 corpuscle pass but a short way. 



As the bone extends in circumference, it also increases in thickness ; the 

 vacuities between the bony spicula become narrowed or disappear, and at a 

 more advanced period the tabular bones of the cranium are tolerably compact 

 towards the centre, although their edges are still formed of slender radiating 

 processes. At this time also numerous furrows are grooved on the surface 

 of the bone in a similar radiating manner, and towards the centre these are 

 continued into complete tubes or canals in the older and denser part, which 

 run in the same direction. The canals, as well as the grooves, which become 

 converted into canals, contain blood-vessels supported by processes of the 

 investing membrane, and are lined with granular cells, which deposit con- 

 centric layers of bone inside these channels ; and, when thus surrounded with 

 concentric laminae, these tubular cavities are in fact the Haversian canals. 



Fig. LI. I may here observe that in earlier stages, such as that 



shown in fig. XLIX., vessels may be seen in the soft tissue, some 

 twice or three times the size of a blood-capillary, others con- 

 siderably more, but all with only a homogeneous coat with 

 cells upon it here and there, and without a muscular layer. 



Ossification in cartilage. It has already been stated 

 that, in by far the greater number of bones, the primitive 

 soft cellular matter of which they originally consist is 

 very quickly succeeded by cartilage, in which the 

 ossification begins. One of the long bones taken from 

 a very small embryo, just before ossification has com- 

 menced in it, is observed to be distinctly cartilaginous. 

 In the tibia of a sheep, for example, at a time when 

 the whole embryo is not more than an inch and a 

 quarter in length, we can plainly see that the substance 

 consists of cartilage-cells imbedded in a pellucid matrix. 

 These cells, which can scarcely be said to be collected 

 into groups, are much larger in the middle part of the 

 shaft where ossification afterwards commences, and 

 there also they are mostly placed with their long 

 diameter across the direction of the bone : towards the 

 ends they are much smaller and closer together, and 

 the cartilage there is less transparent. As it enlarges 

 the cartilage acquires firmer consistence ; it represents 

 in figure the future bone, though of course much 

 smaller in size, and it is surrounded with a fibrous 

 membrane or perichondrium, the future periosteum. 

 Vessels ramify in this membrane, but none are seen iu 

 the cartilage until ossification begins. 



In a long bone the ossification commences in the 

 middle and proceeds towards the ends, which remain 

 long cartilaginous, as represented in fig. u. At 

 length separate points of ossification appear in them, and form epiphyses, 

 which at last are joined to the body of the bone. 



The newly formed osseous tissue is red and obviously vascular, and blood- 

 vessels extend a little way beyond it into the adjoining part of the cartilage. 



Fig. LI. HUMERUS 

 OF A FCETUS, NATU- 

 RAL SIZE. 



The upper half is 

 divided longitudinal- 

 ly, a, cartilage, b, 

 bone, which termin- 

 ates towards the car- 

 tilage by a slightly 

 convex surface. 



